EPA-600/2-77-027
February 1977
Environmental Protection. Technology Series
DEVELOPMENT OF A PORTABLE DEVICE TO
COLLECT SULFURIC ACID AEROSOL
Interim Report
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park,. North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-027
February 1977
DEVELOPMENT OF A PORTABLE DEVICE TO
COLLECT SULFURIC ACID AEROSOL
Interim Report
by
William J. Barrett, Herbert C. Miller,
Josiah E. Smith, Jr., and Christina H. Gwin
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
Contract No. 68-02-2234
Project Officer
Kenneth J. Krost
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences
Research Laboratory, U.S. Environmental Protection Agency, and
approved for publication. Approval does not signify that the con-
tents necessarily reflect the views and policies of the U.S.
Environmental Protection Agency, nor does mention of trade names
or commercial products constitute endorsement or recommendation
for use.
11
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PREFACE
The Environmental Protection Agency has a need for a better
method than is presently available to determine concentrations of
sulfuric acid in the ambient atmosphere. This need exists be-
cause of the recognition of the potentially harmful health effects
of sulfuric acid and sulfates generated by burning increased
amounts of coal and particularly by the use of catalytic converters
in automobiles.
In research on this analytical problem under this contract
and other closely related contracts emphasis has been placed on
the use of a Teflon filter as a sampling device and the ultimate
use of a sulfur-specific flame photometric detector for measuring
the sulfur evolved on heating the filter. This contract was con-
cerned primarily with sampling aspects of the overall method. The
principal task was an investigation of the possible interfering
effects of other gaseous and particulate atmospheric pollutantsA ~~
on the"collection of sulfuric acid on a filter. Although several
kinds of analytical procedures, including the flame photometric
technique, were used in this investigation to measure sulfuric
acid, the study did not encompass all details of a complete
sampling and analytical method; instead, it was limited to the
evaluation of a filter as a sampling device, especially to studies
of interactions between collected sulfuric acid aerosol particles
and other simultaneously occurring pollutants.
111
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ABSTRACT
The purpose of this investigation was to determine the effects
of possible atmospheric interferents on the quantitative collec-
tion of sulfuric acid aerosol on a filter.
r
Sulfuric acid aer_osol was generated in the laboratory with a
flame atomizer and collected on Teflon filters. The filters were
exposed to potential gas and vapor interferents and to particulate
interferents during, before, or after the collection of the sul-
furic acid.
Measurements of sulfuric acid were made by an acid-base indi-
cator method or by extraction with benzaldehyde and titration.
Also, sulfur evolved on heating the filters was measured by the
flame photometric method.
Ammonia, particulate calcium carbonate, and ambient particu-
late material (collected near a busy street) caused severe losses
of sulfuric acid; particulate ferric oxide and silicate clay
caused an intermediate loss; pyridine and phenol vapors, particu-
late fly ash, and so.ot caused little or no loss; and sulfur diox-
ide and nitrogen dioxide had no effect (in the absence of other
materials).
IV
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CONTENTS
Preface .................................................. iii
Abstract ............................. .................. . . iv
Figures .................................................. vi
Tables [[[
1 . Introduction ................ ; ....................... 1
2. Conclusions and Summary of Results .................. 3
Results of experiments using the Bromophenol Blue
Method of Determining Total Acid ........ ........ 3
Results of Experiments using the Benzaldehyde
Extraction Method of Determining Sulfuric Acid. . 4
Results of Experiments Involving Flame Photometric
Detection of Sulfur Evolved from Heated Mitex
Filters ......................................... 5
3 . Recommendations ..................................... 7
4. Experimental Apparatus and Procedures ............... 9
Sulfuric Acid Aerosol Generator ................... 9
Construction and Operating Parameters ......... 9
Particle Size ................................. 11
Flame Temperature ............................. 11
Background Contamination ...................... 13
Sulfur Dioxide in Generator Output ............ 15
Analytical Methods ................................ 16
Bromophenol Blue Method ....................... 16
Barium Chloranilate Method .................... 17
Benzaldehyde Extraction Method ..... ........... 17
Other Analytical Methods ...................... 18
Selection of Filter Material ...................... 19
Flame Photometric Detection of Volatilized
Sulfuric Acid ................................... 20
Apparatus ............................... ...... 20
Sample Loading Procedure ...................... 22
Calibration with Known Amounts of Sulfuric
Acid ........................................ 22
Comparison of Flame Photometric and Benz-
aldehyde Extraction Procedures .............. 25
5. Results of Interference Studies ..................... 26
Gas and Vapor Inter ferents ........................ 26
Introduction of Gases Directly into Sulfuric
Acid Aerosol ......... . ...................... 26
Introduction of Gases and Vapors onto Filters
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Particulate Interferents 35
Analyses by Bromophenol Blue and Barium
Chloranilate Methods 35
Analyses by the Benzaldehyde Extraction Method 40
Analyses with a Flame Photometric Detector.... 49
References 58
FIGURES
Number Paqe
1 Sulfuric Acid Aerosol Generator 10
2 Schematic Diagram of Volatilization-Flame Photometric
Apparatus 21
3 Flame Photometric Response to Standard Samples of
Sulfuric Acid. Temperature, 200°C 24
4 Apparatus for Exposure of Preloaded Filters to
Ammonia 29
5 Modified Apparatus for the Exposure of Preloaded
Filters to Ammonia 29
6 Effect of Predeposited Ambient Particulate Matter on
Flame Photometric Detection of Sulfuric Acid.
Temperature, 200°C 50
7 Effect of Predeposited Ambient Particulate Matter on
Flame Photometric Detection of Sulfuric Acid.
Temperature, 150°C 52
8 Effect of Hold Time on Flame Photometric Response.
Teflon Sample Disks Loaded with Sulfuric Acid and
Ambient Particulate Matter 53
9 Flame Photometric Response to Sulfuric Acid Alone or
to Calcium Carbonate Alone. Temperature, 200°C.... 55
10 Effect of Calcium Carbonate on Flame Photometric
Detection of Predeposited Sulfuric Acid.
Temperature, 200°C 56
11 Effect of Predeposited Calcium Carbonate on Flame
Photometric Detection of Sulfuric Acid.
Temperature, 200°C. 57
VI
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TABLES
Number Page
1 Size of Sulfuric Acid Aerosol Particles 12
2 Data Illustrating Background Contamination of Sul-
furic Acid Aerosol Collected on Mitex Filters 13
3 Measurements of Acidity and Total Sulfate on Mitex
Filters Collecting a 300-yg/m3 Sulfuric Acid
Aerosol 15
4 Calibration Data for the Flame Photometric Detector.. 23
5 Results of Experiments in which Sulfuric Acid Aerosol
was Collected on Mitex Filters in the Presence of
Sulfur Dioxide 27
6 Results of Experiments in which Sulfuric Acid Aerosol
was Collected on Mitex Filters in the Presence of
Nitrogen Dioxide 28
7 Effect of Ammonia on Predeposited Sulfuric Acid
Aerosol on Mitex LS Filters 32
8 Effect of Pyridine Vapor on Predeposited Sulfuric
Acid on Mitex LS Filters 34
9 Effect of Phenol Vapor on Predeposited Sulfuric Acid
on Mitex LS Filters 36
10 Effect of Ambient Air on Sulfuric Acid-Spiked Filters 37
11 Effect of Predeposited Ambient Particulate upon the
Collection of Sulfuric Acid Aerosol on Mitex LS
Filters 38
12 Effect of Collected Calcium Carbonate Aerosol upon
Predeposited Sulfuric Acid on Mitex LS Filters 39
13 Effect of Collected Ferric Oxide Aerosol upon Pre-
deposited Sulfuric Acid on Mitex LS Filters 40
VI1
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Number Page
14 Effect of Predeposited Ambient Particulate Material
upon Sulfuric Acid Collected on Mitex LS Filters... 41
15 Effect of Calcium Carbonate Aerosol upon Predeposited
Sulfuric Acid on Mitex LS Filters 42
16 Effect of Predeposited Calcium Carbonate upon Sul-
furic Acid Aerosol Collected on Mitex LS Filters... 43
17 Effect of Predeposited Ferric Oxide upon Sulfuric
Acid Aerosol Collected on Mitex LS Filters 45
18 Effect of Predeposited Fly Ash upon Sulfuric Acid
Collected on Mitex LS Filters 46
19 Effect of Predeposited Soot upon Sulfuric Acid
Collected on Mitex LS Filters 48
20 Effect of Predeposited Clay upon Sulfuric Acid
Collected on Mitex LS Filters 49
Vlll
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SECTION 1
INTRODUCTION
The increasing use of sulfur-bearing coal for the production
of electrical energy, the generation of significant quantities of
sulfuric acid aerosol by catalytic converters in automobiles, and
the appearance of new information on the irritant effects of sul-
furic acid and sulfates, are factors that have combined to inten-
sify interest in the measurement of ambient sulfuric acid aerosol,
The emission of sulfuric acid by automobiles is a problem of
special interest because of the fairly high concentrations found
in the immediate vicinity of roadways and the small size of the
sulfuric acid particles generated.
The potentially harmful health effects stem from certain
unique properties of sulfuric acid aerosol. Sulfuric acid is the
most severe bronchial and lung irritant among the various species
of sulfur compounds that can occur in the atmosphere. The parti-
cles derived from automobiles have a mass mean diameter of only a
few hundredths of a micrometer and therefore can penetrate deeply
into the lung and be deposited in the alveoli. These small parti-
cles are relatively highly concentrated in terms of the normality
of the sulfuric acid and are therefore stronger irritants than
particles that have grown in size over a period of time by coagu-
lation and absorption of water vapor.
In order to assess the health effects potential of sulfuric
acid under these circumstances, it is necessary to have sampling
and analytical methods that will measure sulfuric acid concentra-
tions without interference or artifacts arising from the presence
of other substances in the sampled atmosphere or from various '
environmental factors such as relative humidity or temperature.
Because many interfering reactions are possible, especially dur-
ing the collection of a sample, the accurate determination of
sulfuric acid aerosol is a difficult problem that has so far not
been adequately solved.
Although sampling cannot be arbitrarily isolated from the
method of analysis subsequently used to determine the species
sought, there are certain difficulties peculiar to sampling sul-
furic acid aerosol. In general terms, these difficulties may be
said to result from interactions of sulfuric acid with other par-
ticulate and gaseous constituents of the ambient atmosphere or
the sample and from the simultaneous presence in the atmosphere
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of sulfate salts and sulfur dioxide. Reactions of sulfuric acid
with copollutants have been shown to occur on filters, the collec-
tion medium that is generally best suited for sampling atmo-
spheric particulate material. The oxidation of sulfur dioxide to
sulfate also occurs on filters under certain conditions. These
phenomena—and others that may be expected to occur—often result
in failure of attempts to measure ambient concentrations of sul-
furic acid because some or all of the acid is lost during the
sampling process. Futhermore, methods of analysis that involve
dissolving the sulfuric acid along with other soluble constitu-
ents of the sample introduce errors resulting from reactions that
occur in the solvent medium; a sampling method that will permit
subsequent processing and -analysis of the sample without loss (or
addition) of sulfuric acid is therefore desirable.
Work on this contract has been concerned principally with
analytical evaluation of possible interfering effects that may
occur during collection of a sample on a filter. During the
early months of the contract period efforts were made to estab-
lish techniques that would permit studies to be conducted with a
submicron aerosol of sulfuric acid at a concentration (10 yg/m3)
near reported ambient levels. In accordance with the Scope of
Work, it was intended that interference studies would be con-
ducted by introducing the potential particulate and gaseous inter-
ferents directly into the aerosol, with subsequent collection of a
sample and analysis. However, inability to obtain a clean aero-
sol free of significant levels of background ammonia and amines,
nitrogen dioxide, 'and particulate material finally led to the
abandonment of this-approach. Instead, an aerosol with a concen-
tration of about 300 yg/m3 was used. At this concentration level,
the ratio of sulfuric acid generated to the volume of dilution
air used is such that the relative amounts of background interfer-
ents are reduced to less significant levels. Instead of mixing
the interferent materials to be studied directly into the aerosol,
30- to 50-yg amounts of sulfuric acid were deposited on a filter
in 10 to 20 min and the interferent gas or particulate material
was then passed through or deposited on the filter. In some
instances, particulate interferents were added to the filter prior
to the collection of sulfuric acid.
This report describes the techniques used to generate the
sulfuric acid aerosol; the analytical methods used to measure sul-
furic acid, total sulfate, and several interferent materials; and
the apparatus and procedures used in studies of the flame photo-
metric detection of sulfur evolved from heated filters. It gives
the results of experiments designed to determine the effects of
various potential interferents on the collection of sulfuric acid
on filters, and summarizes the conclusions reached.
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SECTION 2
CONCLUSIONS AND SUMMARY OF RESULTS
The collection of sulfuric acid aerosol on Teflon filters, as
a first step in the measurement of ambient sulfuric acid concentra-
tions, is subject to interference by alkaline gases and by parti-
culate materials in the sample air. Strong interference is caused
by gaseous ammonia, particulate calcium carbonate, and ambient
particulate material from an urban environment; an intermediate
degree of interference is caused by ferric oxide and a silicate
clay soil; and relatively little or no interference is caused by
nitrogen dioxide, sulfur dioxide, fly ash, soot, and the vapors of
pyridine and phenol. In the presence of ambient particulate
material sulfuric acid or sulfate appears to be reduced slowly to
sulfur dioxide when heated on a Teflon filter. These conclusions
are based on the experimental evidence summarized in the following
paragraphs. This evidence suggests that the determination of
atmospheric sulfuric acid by collection of a sample on a filter
over a period of time is subject to error from interfering re-
actions that may occur during sampling.
RESULTS OF EXPERIMENTS USING THE BROMOPHENOL BLUE METHOD OF
DETERMINING TOTAL ACID
• Mitex filters on which 50 yg of sulfuric acid had been de-
posited as a single drop of standard solution were exposed
to ambient air in an industrial area at 15 l/min for 10,
30, or 60 min. No losses of sulfuric acid were observed
in 10 or 30 min, but in 60 min about 25% of the acid was
neutralized, as measured by the bromophenol blue method.
• In the absence of other contaminants, sulfur dioxide added
directly to sulfuric acid in the aerosol phase did not
affect the amount of acid collected on a Mitex filter, as
measured by the bromophenol blue method, or the amount of
total sulfate, as determined by the barium chloranilate
method.
• In the absence of other contaminants, nitrogen dioxide
added directly to the sulfuric acid aerosol had no effect
on the amount of acid or total sulfate on a Mitex filter,
as measured by the bromophenol blue and barium chloranilate
methods.
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• When a Mitex filter that had been preloaded with 0.45 ymol
of sulfuric acid from the aerosol generator was exposed to
a known amount (0.14 ymol) of ammonia in air, approximately
0.1 ymol of the sulfuric acid disappeared, as measured by
the bromophenol blue method.
• Mitex filters upon which various amounts of sulfuric acid
(20 to 80 yg) had been predeposited were exposed to an
aerosol of finely divided ferric oxide. The average re-
covery of sulfuric acid was about 75% by the bromophenol
blue method and the average recovery of total sulfate was
about 95% by the barium chloranilate method.
• Mitex filters upon which 40 to 70 yg of sulfuric acid had
been predeposited were exposed to an aerosol of finely
divided calcium carbonate. No acid or soluble sulfate was
recovered from these filters on analysis by the bromophenol
blue and barium chloranilate methods. The sulfuric acid
appeared to be completely converted to calcium sulfate.
• Mitex filters were exposed first to laboratory ambient
particulate material and then to sulfuric acid aerosol.
Upon analysis by the bromophenol blue and barium chloranilate
methods, about 50% of sulfuric acid was recovered, while all
of the total-sulfate was recovered. Some neutralization
of the s>ulfuric acid yielding a soluble sulfate appeared
to have occurred.
RESULTS OF EXPERIMENTS USING THE BENZALDEHYDE EXTRACTION METHOD
OF DETERMINING SULFURIC ACID
• The benzaldehyde extraction method was confirmed as specific
for sulfuric acid in the presence of ammonium bisulfate,
ammonium sulfate, and calcium sulfate. Sulfuric acid was
extracted from filters with dry, freshly distilled benzal-
dehyde, then extracted from the organic solvent with water,
and finally measured with a microadaptation of the barium-
Thorin titration method.
• Measured amounts (35 to 50 yg) of sulfuric acid were de-
posited on Mitex filters from the aerosol and the filters
were then exposed to measured quantities of ammonia
(2 to 40 yg) in air. The filters were analyzed for sulfuric
acid and ammonium ion and the gas passing through the fil-
ters was analyzed for unabsorbed ammonia. The results of
these measurements showed.that when sulfuric acid was
stoichiometrically in excess, all of the ammonia was col-
lected on the filter and the excess acid was measurable
as HaSOi*; when ammonia was stoichiometrically in excess,
all of the sulfuric acid was neutralized and the excess
ammonia was found in the effluent gas passing through the
filter.
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• The experiments with calcium carbonate and ferric oxide in
which the sulfuric acid was determined by the bromophenol
blue method were repeated with analysis by the benzaldehyde
extraction method. Whether the sulfuric acid was deposited
on the filter before or after the collection of calcium
carbonate aerosol, losses of sulfuric acid generally in
excess of 75% were observed. When filters containing pre-
deposited iron oxide aerosol were exposed to sulfuric acid,
approximately 95% of the sulfuric acid was recovered.
• Similar experiments were conducted with one sample of fly
ash from a coal-burning electric power plant and one sample
of soot deposited from a fuel-rich acetylene-air flame.
No evidence of loss of significant amounts of sulfuric acid
was observed with either material.
• Samples of predeposited ambient particulate material col-
lected on Mitex filters near a busy street were exposed to
sulfuric acid aerosol. Analyses of the filters by the
benzaldehyde extraction method indicated that only 30 to 40%
of the sulfuric acid could be recovered.
• Efforts to conduct similar experiments with predeposited
calcium silicate were not successful, apparently because
the strong alkalinity of the calcium silicate interfered
with the analysis by the benzaldehyde extraction method.
However, experiments with a clay soil containing silicates
were successful and indicated that slightly more than 50%
of the added sulfuric acid was recovered.
• Measured amounts of vapor of the organic amine pyridine
were exposed to filters preloaded with known amounts of
sulfuric acid collected from the aerosol phase as in the
experiments with ammonia. However, analyses of the filters
showed no significant losses of sulfuric acid with pyridine
as opposed to the stoichiometric losses observed with
ammonia.
• Analyses following exposures of collected sulfuric acid to
measured amounts of phenol vapor also showed no significant
losses of the acid.
RESULTS OF EXPERIMENTS INVOLVING FLAME PHOTOMETRIC DETECTION OF
SULFUR EVOLVED FROM HEATED MITEX FILTERS
• A Meloy Sulfur Gas Analyzer was modified for these experi-
ments, principally by connecting the detector to a glass
tube in an aluminum block in which Mitex .filter disks
could be heated in a controlled manner so that volatilized
materials could be flushed with a stream of air into the
flame. The response of the detector was calibrated with
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known amounts (1 to 3 yg) of sulfuric acid deposited on
filters and evaporated in the heated block.
• When sulfuric acid aerosol was collected on filters con-
taining predeposited ambient particulate material and de-
termined by heating the filter at 200°C, the observed
flame photometric detector response was much less than the
response to the same amount (2 yg) of sulfuric acid alone.
Moreover, the smaller initial and immediate peak response
characteristic of sulfuric acid was followed by a gradually
increasing response over a period of about 1 min and then
a slowly decreasing response over 5 to 10 min. These
results suggested that the reduced initial response may
have resulted from loss of sulfuric acid by reaction with
constituents of the ambient particulate material, and that
the slowly decreasing response may have resulted from re-
duction of sulfate to sulfur dioxide.
• Similar experiments conducted at 150°C instead of 200°C
gave results that were similar, except that the responses
were further attenuated because sulfuric acid was vola-
tilized from the filters more slowly.
• At each temperature, the ambient particulate material
alone (without any added sulfuric acid) gave only a small
initial response, suggesting the presence of a small
amount of ambient sulfuric acid. At 200°C, this small
initial response was followed by the same type of gradually
increasing and decreasing response observed when sulfuric
acid was added, again suggesting the reduction of sulfate
to sulfur dioxide by constituents of the ambient particu-
late material. At 150°C, the initial peak was smaller and
the gradually increasing and decreasing response was not
evident.
• Similar experiments conducted with predeposited calcium
carbonate aerosol gave the same results that were found
with the bromophenol blue and benzaldehyde extraction
methods of analysis; that is, the sulfuric acid was ap-
parently completely neutralized and no sulfur flame photo-
metric detector response was observed. The absence of any
slow response in this instance presumably resulted from
the absence of any substance that could reduce sulfate to
sulfur dioxide at 200°C.
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SECTION 3
RECOMMENDATIONS
The results of the experimental work described in this report
indicate that intensified effort should be directed toward fixa-
tion of sulfuric acid by an appropriate reaction in the aerosol
phase or in or on the collection medium. Such a reaction should
produce a derivative in an amount directly proportional to the
amount of sulfuric acid in the air sample. The derivative itself
should be measurable by a suitable analytical method without
interference from other constituents of the air sample. Research
is already underway elsewhere with this objective. The conclu-
sions and results given in the previous section suggest that that
research should be continued and expanded.
The Environmental Protection Agency is currently supporting
the development of a sulfuric acid analyzer based on the collec-
tion of sample on a filter followed by volatilization and measure-
ment with a flame photometric detector. This principle has certain
strong potential advantages over other methods, particularly in
sensitivity; however, the results of the present study show that
flame photometric measurement of the sulfur volatilized from a
particulate sample collected on a filter may not yield a response
proportional to the ambient concentration of sulfuric acid.
Further studies should be conducted to optimize this technique and,
especially, to adapt it to the measurement of a derivative formed
by" a suitable fixation reaction.
In any further research of the type described in this report
particular attention should be given to the following experimental
tasks :
• Determine whether sulfuric acid aerosol generator systems
other than the burner-aspirator used in this work—for
example, ultrasonic generators—can be used to produce a
clean aerosol free of background contaminants while main-
taining the desired small particle size and allowing con-
venient and accurate introduction of potential interfer-
ents directly into the aerosol.
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• Conduct a systematic evaluation of filter materials from
which sulfuric acid (or another sulfur compound) can be
volatilized for flame photometric detection.
• Investigate in depth the possible interference of sulfur
dioxide and other sulfur compounds in the presence of com-
plex ambient particulate material when the volatilization-.
flame photometric method is to be used for detection. t
• Optimize the conditions and equipment for volatilization-
flame photometric detection, especially the effects of
temperature, rate of heating, and types of surfaces exposed;
identify and quantitate the sulfur species evolved on
heating.
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SECTION 4
EXPERIMENTAL APPARATUS AND PROCEDURES
SULFURIC ACID AEROSOL GENERATOR
Construction and Operating Parameters
A generator of sulfuric acid aerosol similar to the one de-
scribed by Thomas et al. was constructed.l This system was se-
lected because it was reported to have certain attributes that
were considered to be desirable for the purposes of this project.
In particular, no other .generator system could be expected to.pro-
duce predominantly submicron particles, while simultaneously per-
mitting flexibility for the addition of potential gaseous and
particulate interferents directly to the aerosol. It also allowed
easy adjustment of concentrations, flow rates, and other param-
eters. Furthermore, it was reported to generate no sulfur di-
oxide. Several problems, however, were encountered, and a sub-
stantial amount of effort was devoted to attempts to correct these
difficulties.
The generator is illustrated in Figure 1. Its principal
component was a Beckman No. 4020 atomizer-burner situated at the
base of a 1.22-m by 99-mm i.d. Pyrex chimney. The fuel gas was
hydrogen and the oxidizer was a mixture of oxygen and argon. A
dilute solution of sulfuric acid was aspirated into the flame
from a beaker placed underneath the burner. Filtered air was fed
into a metal box placed under the bottom of the chimney and en-
closing the burner. This air was forced into the box by a vacuum
cleaner type blower so that excess filtered air shrouded the
burner and bottom end of the chimney. The ends of two 13-mm i.d.
glass probes were located close together about 10 cm below the
top of the chimney. One of these probes had a side arm closed
with a silicone septum through which gases could be added at known
flow rates. The probes were connected to stainless steel holders
for 47-mm filters, which were followed by 6-1/min critical ori-
fices and a small Cast pump. One side of this dual sampling
system was always used as the reference for determination of the
concentration of the sulfuric acid aerosol, while the other side
was used for the introduction of interferent gases or for holding
a filter containing predeposited particulate material. The
excess aerosol from the top of the chimney was exhausted through
a hood.
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EXHAUST
FLAME
SULFURIC
ACID
SOLUTION
ONE OF TWO
SAMPLING PROBES
^ 1 /
1 hJ
1 1 SEPTUM
^•^•M
,
PUMP
FILTER
HOLDER
PYREX
-CHIMNEY
AIR IN
PREFILTER
HYDROGEN
OXYGEN + ARGON
Figure 1. Sulfuric Acid Aerosol Generator (not to scale)
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The principal operating parameters for the aerosol generator
and their usual values were as follows:
« Hydrogen fuel gas: 15 nunHg (positive pressure) /
• Oxygen-argon mixture: Oz , 105 mmHg; Ar, 672 mmHg
• Sulfuric acid solution concentration: 0.002 N
• Sulfuric acid aspiration rate: 2 ml/min • •-,•-'"
• Temperature at inlet of probe: 35°C
• Temperature at filter: 30°C
• Sampling flow rate: 6 1/min
All of these parameters were varied experimentally in order to
find the optimum set of conditions.
Particle Size
An estimate of the particle-size distribution of the aerosol
is given in Table 1. The conditions existing at the time these
measurements were made were not identical to those given above,
but the results are believed to be representative. Measurements
were made with a Thermo-Systerns Electrical Aerosol Size Analyzer
(Model 3030) and with a modified Climet Particle Analyzer. Samples
were taken at a position in the sampling system that corresponded
to the position of the filter holder. Samples were also taken
after the filter holder with Mitex LS filters in place, and at
several positions in the metal box at the base of the chimney.
The majority of particles at a position near the location of
the filter holder were found to be in the size range of
0.3 to 0.005 ym; 98% of the mass of the particles was in this
range. Slippage of the small particles through Mitex LS filters
was about 3% while slippage of particles larger than 0.25 ym was
determined to be less than 0.1%. The filtered air entering the
metal box was found to contain about 8.4 x 103 particles/nr that
were greater than 0.25 ym; this was less than 0.1% of the concen-^
tration of particles in this size range found in the effluent of " -—•.
the generator. The dilution air also contained negligible concen-
trations in the size range below 0.25 ym. '
Flame Temperature
Initially, when the aerosol generator was operated under the !
conditions described by Thomas et al., little or no sulfuric acid
was found on Mitex LS filters in the sampling line. Much larger
quantities were found in Greenburg-Smith backup impingers contain-
ing 80% isopropanol. This observation indicated that most of the
11
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TABLE 1. SIZE OF SULFURIC ACID AEROSOL PARTICLES
Thermo-Systems Electrical
Aerosol Size Analyzer
Size range/ ym
1 to 0.3
0.3 to 0.005
Number of particles/m3
Volume, cnr/m3
Mass, yg/m3
Volume fraction
1.2 x 107
6.6 x 10~7
1.1
0.02
>1.2 x 10 1!*
>2.7 x 10~5
>45
>0.98
Slippage through Mitex LS filter,
number of particles/m3
3.3 x 10
1 2
Size
Climet Particle Analyzer Range, ym
No. of Particles/nr
;V >1.27 '-i
>0.77
>0.25
8.5 x 102
2.9 x 10s
1.3 x 107
Slippage through Mitex LS
filter, number of parti-
cles/m3
0.25 ym.
8.5 x
The density of the aerosol particles was assumed to be
1.66 g/cm3 (74% H2SOiJ .
sulfuric acid was in the form of vapor and that the temperature of
the gas at the filter was too high. By reducing the pressure of
the hydrogen fuel from the recommended value of 52 mmHg to 15 mmHg
and that of oxygen from 646 mmHg to 517 mmHg, the temperature at
the inlet of the probe was reduced from 200°C to about 80°C. With
these conditions, and with the aspiration of 0.002 N sulfuric acid
at 0.8 ml/min, quantities of sulfuric acid appropriate for analysis
were collected on filters from an aerosol of 40 to 50 yg/m3 in
2 to 3 hr with a sampling flow rate of 10 to 15 1/min.
Later, when experiments involving the addition of nitrogen
dioxide to the aerosol were undertaken, it was found that the
generator system produced nitrogen dioxide concentrations of 1 to
2 mg/m3 (0.5 to 1 parts per million) at the point where the filter
was located in the sampling line. It was felt that this concentra-
tion level was much too high for the types of interference experi-
ments planned. Therefore, the composition of the burner gases
was further modified to reduce the flame temperature by mixing a
relatively large proportion of argon with the oxygen. With a
hydrogen pressure of 15 mmHg, an oxygen pressure of 105 mmHg, and
an argon pressure of 672 mmHg, the temperature of the gases at the
probe inlet was reduced to about 35°C and the concentration of
nitrogen dioxide at the filter was reduced to about 0.7 mg/m3
(0.04 parts per million).
12
-------
Background Contamination
In an effort to circumvent problems arising from the presence
of sulfate particulate and alkaline gases in the dilution air,
the generator was further modified to filter the air that entered
the metal box at the base of the chimney. The extent of background
contamination is illustrated by the data in Table 2. These data
were obtained with the generator operating under the following
conditions: pressure of hydrogen, 15 mmHg; pressure of oxygen,
517 mmHg; concentration of sulfuric acid solution, 0.001 N;
aspiration rate, 0.8 ml/min; temperature at filter, 45°C; sampling
rate, 15 1/min; and sampling time, 150 min. Three of the sixteen
filters were analyzed for acidity by the bromophenol blue method
and the rest were analyzed for total sulfate by the barium chlor-
anilate method.
TABLE 2. DATA ILLUSTRATING BACKGROUND CONTAMINATION OF
SULFURIC ACID AEROSOL COLLECTED ON MITEX FILTERS
Concn of sulfate in-aerosol,
as HaSOit, yg/m3
Dilution air
Unfiltered air
average
Filtered air
average
Sample
filter
29
32
31
1°
—
29
2oc
lc
—
Reference
filter
31
32
32
32
32
28
25
26
26
26
Room air was used for dilution of the aerosol.
Air filtered through the system described in the text was used
for dilution of the aerosol. /*
cThese analyses were conducted by the bromophenol blue method for
the determination of acidity. All other analyses were conducted
by the barium chloranilate method for total sulfates.
X."
13
-------
The filtering system is illustrated in Figure 1. It con-
sisted of a glass wool prefilter, a Lamb Electric Model 11250
vacuum cleaner blower, a Mine Safety Appliances Ultra Aire
No. 82177 absolute filter, and an open-top metal box 25-cm i.d.
by 30-cm high. A positive flow of filtered air out of the top of
the metal box prevented unfiltered room air from entering the
chimney.
The data in Table 2 show that agreement between total sulfate
concentrations was good when two filters were collecting samples
simultaneously (sample and reference filters). However, the
measurements of acidity on three of the filters suggested that all
of the sulfuric acid had been essentially neutralized by alkaline
gaseous substances in either the filtered or the unfiltered air,
presumably by ammonia or amines. Also, the difference between the
averages of total sulfate found with filtered and unfiltered air
indicated that the background air contained about 6 yg/m3 of
particulate sulfate.
The filtering system was then further modified by adding a
scrubber to remove alkaline gases. The scrubber consisted of a
4.3-cm Pyrex tube filled to a depth of 24 cm with small Berl
saddles that had been impregnated with phosphoric acid. When
Mitex filters exposed to the aerosol generated in this system were
analyzed for acidity and for ammonium ion, it was found that the
phosphoric acid scrubber was partially effective in reducing the
background concentration of ammonia.
At this point a decision was made to abandon efforts to work
with an aerosol at a concentration of 30 to 40 yg/m3, or less,
and to undertake a modified approach based on the use of an
aerosol at a concentration high enough so that the background
concentrations of contaminants would be negligible compared with
the "concentration of sulfuric acid. In this modified approach
the concentration of sulfuric acid was increased to about 300 yg/m3
and the sampling time was reduced to 10 to 20 min. In this way
the total quantity of dilution air was reduced by a factor of
about 20, and the total amounts of the background contaminants
reaching the Mitex filters were correspondingly reduced. •'•''•<'
-------
TABLE 3. MEASUREMENTS OF ACIDITY AND TOTAL SULFATE ON
MITEX FILTERS COLLECTING A 300-yg/m3 SULFURIC
ACID AEROSOL
Amount of HaSOi* deposited
Experiment
No.
1
2
3
4
5
6
7
Concn of HaSOi,
feed solution,
equiv ./I
0.02
0.01
0.005
0.005
0.005
0.005
0.005
Sampling
time,
min
10
10
15
10
10
10
10
on filter, yg
Sample
filter
138
88
75
46
45
48
48
Reference
filterb
188
89
81
48
44
48
51
-
(\
a Acidity determined by the bromophenol blue method.
Total sulfate determined by the barium chloranilate method.
Sulfur Dioxide in Generator Output
Thomas et al.* reported that their sulfuric acid aerosol gen-
erator did not produce detectable amounts of sulfur dioxide as
measured by the West-Gaeke method. A similar result was obtained
on this project when the West-Gaeke method was used. However,
additional experiments showed that the West-Gaeke method is not
applicable to the determination of sulfur dioxide in the combus-
tion products of the hydrogen-oxygen flame.
When a known amount of sulfur dioxide gas was introduced into
the aerosol generator through the septum in the sampling probe
(see Figure 1) with the burner off, it could be measured satisfac-
torily. That is, the amount of sulfur dioxide found at the point
corresponding to the location of the filter holder was equal to
the amount introduced through the septum. However, when the
burner was on and the combustion products of the flame were pres-
ent, no sulfur dioxide could be detected at the location of the
filter holder. Also, when the burner was on, and the tetrachloro-
mercurate absorbing solution was spiked with known amounts of
sodium bisulfite, either before or after sampling, no sulfur
dioxide was detected. In these analyses, procedures recommended
for the elimination of interference by nitrogen dioxide, ozone,
and heavy metals were followed. These experiments indicated that,
whether or not sulfur dioxide was added to the generator gas, it
could not be detected because of interfering reactions that
occurred in the tetrachloromercurate reagent solution.
15
-------
In further experiments, sulfur dioxide was injected into the
sampling line through the septum and measured at the point corre-
sponding to the downstream location of the filter by means of gas
chromatography with a flame photometric detector. With the
burner off, the expected amount of sulfur dioxide was found; with
the burner on, no sulfur dioxide was detected. This result sug-
gested that sulfur dioxide was rapidly oxidized in the presence
of the combustion products of the flame, presumably by the nitro-
gen dioxide generated in the flame. It is possible, therefore,
that the small difference between acidity and total sulfate (as
H2SOi») shown in Table 3 may have been the result of oxidation of
background sulfur dioxide.
Thus, even though Thomas e_t al. would not have detected sul-
fur dioxide in the aerosol by the West-Gaeke method if it had
been present, they apparently were correct in stating that it was
not present. These findings made questionable the use of the
hydrogen-oxygen flame generator in studies of the effect of intro-
ducing sulfur dioxide directly into the sulfuric acid aerosol.
ANALYTICAL METHODS
The principal analytical methods used in this work were the
bromophenol blue method for the determination of acidity,2 the
barium chloranilate method for the determination of total sul-
fate, 3/lf and the benzaldehyde extraction method5 coupled with a
barium-Thorin titration procedure6 for the specific determination
of sulfuric acid. The procedures described in the literature
were modified somewhat to meet the needs of this project.
The bromophenol blue and barium chloranilate methods were
selected for use in the early stages of the project. These
methods were effective in most of the instances in which they
were applied, but both had certain limitations. The bromophenol
blue method is useful only for determination of total acidity and
does not distinguish strong acids from most weak acids; also, the
sensitivity and precision are less than were desired for this
work. The barium chloranilate method was found to be subject to
interference by sulfite ion, a point not adequately covered in the
literature. The detailed procedures for these methods are de-
scribed in the following paragraphs.
Bromophenol Blue Method
For determination of acidity by the bromophenol blue method,
filters were cut into 0.5-cm strips and placed in a 10-ml volu-
metric flask containing 2 ml of 80% isopropanol and 1.00 ml of a
0.01% aqueous solution of bromophenol blue. The contents of the
flask were mixed on a vortex mixer, diluted to volume with dis-
tilled and deionized water, and mixed again. The absorbance of
the solution, which decreased nonlinearly with increasing acidity,
was measured at 592 nm in a 1-cm cell with a Beckman Model DU
16
•>
-------
spectrophotometer. Distilled water was used in the reference
cell. A calibration curve was prepared by an identical procedure
with known volumes of standard solutions of sulfuric acid in the
presence of the filter medium. The method was useful for the
determination of acidity (as HaSOi*) over the range of 5 to 40 yg
per filter without dilution of the sample. However, poor re-
producibility at the limit of detection required that calibration
curves be prepared with each set of samples for analysis by this ^
method.
Barium Chloranilate Method
For the determination of sulfate by the barium chloranilate
method, a filter sample was placed in a 125-ml screw-capped ' /
Erlenmeyer flask containing about 25 mg of barium chloranilate
and 10.00 ml of 80% isopropanol. The sample was then mixed for /
30 min on a rotary shaker, centrifuged, and the absorbance of the
resulting solution measured at 310 nm in a 1-cm fused-silica cell
with a Beckman Model DU spectrophotometer. An 80% isopropanol
solution was used in the reference cell. A linear calibration
curve was prepared with known quantities of standard sulfuric acid
solution that were treated in the same manner as experimental
samples. The useful range of the method was 10 to 50 yg of sul-
furic acid per filter without dilution of the sample. The barium
chloranilate method exhibited better reproducibility than the
bromophenol blue method for determinations of sulfuric acid,
although neither method was specific for sulfuric acid.
Benzaldehyde Extraction Method
It was found that the bromophenol blue and barium chloranilate
methods were not compatible with the benzaldehyde extraction of
sulfuric acid. No meaningful data could be obtained from analyses
of the aqueous extracts of known, dilute solutions of sulfuric
acid in benzaldehyde by either method. It appeared that the
strong uv absorption of benzaldehyde precluded the spectrophoto-
metric measurement of chloranilic acid at 310 nm in the barium
chloranilate method and that the presence of benzoic acid
(pKa - 4.1) in benzaldehyde introduced uncertainties in the acidity d_
measurements by the bromophenol blue method. Attempts to remove '
the residual acidic material from benzaldehyde with an alkaline
wash (aqueous sodium carbonate solution) achieved only limited
success because of the variable and fairly rapid air oxidation of
benzaldehyde to benzoic acid.
Consequently, the microtitration of sulfate with barium
perchlorate employing the Thorin indicator was investigated for
the determination of sulfuric acid extracted from filters with
benzaldehyde. An adaptation of the method reported by Fritz
and Yamamura* was used initially to quantitate the sulfate
present in aqueous extracts of known solutions of sulfuric acid
in benzaldehyde. The results of these preliminary experiments
17
-------
demonstrated a recovery of sulfuric acid of approximately 90%
(including the aqueous extraction step) with a relative standard
deviation of less than 5%. The minimum quantity of sulfuric acid
that could be determined with the method was found to be approxi-
mately 5 yg.
The procedure that evolved for the assay of sulfuric acid on
Mitex filters consisted of the following steps:
A 47-mm filter sample was extracted with 5-ml of freshly-
distilled (dry and colorless) benzaldehyde; the resulting
extract was then centrifuged to separate particulates and a
3-ml portion of the benzaldehyde solution was back-extracted
with 1.5 ml of distilled water; a 1.0-ml aliquot of the
aqueous extract was then treated with a cation-exchange
resin, diluted with 2-propanol, and titrated with a solution
of barium perchlorate in 80% 2-propanol, employing the
Thorin indicator for end-point detection.
In order to establish the selectivity of the benzaldehyde ex-
traction, several filters were spiked with potentially interfering
substances and analyzed by the procedure described above. Solutions
or suspensions of ammonium bisulfate, ammonium sulfate, and
calcium sulfate in methanol were each spiked onto Mitex filters
with a microliter syringe. Because of the residual water content
of the benzaldehyde that was used in the extractions, highly vari-
able results were obtained in the initial experiments. Subsequently
however, consistent results were achieved with the use of freshly-
distilled benzaldehyde stored under dry nitrogen to exclude water
and to minimize air oxidation. Thus, the later results demonstrated
that none of these potential interferents was recovered with benz-
aldehyde extraction in excess of 5% of the amount added to the
filter. The spikes of ammonium bisulfate, ammonium sulfate, and
calcium sulfate covered the range of 15 to 100 yg per 47-mm filter
disk and at this level posed no interference in the specific deter-
mination of small amounts of sulfuric acid.
In subsequent studies of the stability of sulfuric acid aero-
sol deposited on filters with other materials, blank determina-
tions with interferent materials and determinations of sulfuric
acid spikes added to benzaldehyde extracts of filters were
employed to insure the validity of the results.
Other Analytical Methods
Other analytical methods that were used in the course of this
work included the following:
• Ammonia was determined by the indophenol method described
by Harwood and Kuhn.7 This method could not be used for
the determination of ammonia in the generator output be-
cause of interference apparently caused by nitrogen dioxide.
18
-------
• Ammonia was also measured by use of the Orion Model 95-10
ammonia electrode.
• Nitrogen dioxide was determined by the Griess-Saltzmann
method.8
• Sulfur dioxide was measured by the West-Gaeke method9 and
by gas chromatography with a flame photometric detector.
\
SELECTION OF FILTER MATERIAL
No systematic evaluation of filter materials was undertaken
on this project. The experimental work on comparison of filters t..< •/'.
that was undertaken early in the project was somewhat beclouded 'y
by uncertainties in the analytical procedures and in the^ composi-
tion of the generator effluent. Some later data is of interest,
however, and will be described briefly.
Several types of filter materials were considered. They
included Millipore HA filters(mixed cellulose esters, pore size
0.45 ym); Mitex LS filters (Teflon fiber mat, pore size 5.0 ym);
Versapor filters (glass fibers and epoxy resin, pore size 0.9 ym) ;
Fluoropore FH filters (Teflon bonded to a polyethylene scrim, pore
size 0.5 ym); and Nuclepore filters (polycarbonate, pore size
0.4 ym). The Millipore HA filter was recommended for the collec-
tion of sulfuric acid aerosol by Thomas ejt al. l . The Mitex filter
was preferred by Leahy5 but was rated by Thomas as among the
least effective for the ^collection of sulfuric acid. Liu and Lee C.
reported on the efficiencies of several types of filters for
collecting small particles, and found that Mitex LS collected
more than 80% of particles in the size range of 0.03 to 1 ym—in
spite of the relative large pore size of 5.0 ym.10 Glass fiber
filters were not considered for sample collection because of the
well known problems of alkalinity and reactivity with sulfur
dioxide.
Some experiments were conducted to determine the recovery of
sulfuric acid spikes added to several types of filters from a
microsyringe. (These experiments were of interest in connection
with studies involving the use of the bromophenol blue and barium
chloranilate methods of analysis but, of course, were not relevant
to later experiments involving volatilization and flame photometric
measurement of the evolved sulfur compounds.) After measured vol-
umes of diluted sulfuric acid solutions were deposited on the
filters (and on glass slides for control), the filters were
extracted with 80% isopropanol and the sulfate was determined by /
the barium chloranilate method. The recovery was 98% from Mitex LS '
filters, 93% from Millipore HA filters, 87% from Fluoropore FH
filters, and 0% from Versapor filters. Retention of the sulfuric
acid by the Versapor filters suggested that some kind of fixation
19
-------
reaction had occurred and that these filters could possibly be
used as the basis for a selective method of measuring sulfuric
acid aerosol.
The resistance to airflow of 47-mm Mitex filters was found
to be 80 mmHg at a flow rate of 15 1/min; that of Fluoropore FH
filters was 210 mmHg at the same flow rate.
A principal determining factor in the final selection of
Mitex LS filters for use on this project was the fact that this
material was also being used by other Environmental Protection
Agency contractors who were engaged in closely related research
aimed at development of a flame photometric method for determining
sulfuric acid aerosol. Also, the limited evaluations conducted
on this project indicated that the Mitex filters were the best
among those available and tested.
FLAME PHOTOMETRIC DETECTION OF VOLATILIZED SULFURIC ACID
Apparatus
Experiments involving the use of a flame photometric detector
for the estimation of sulfuric acid were conducted with a modified *
Meloy Laboratories Model SA 160-2 Sulfur Gas Analyzer. It was
necessary to make several modifications to the Meloy instrument
and to the sample flow system before acceptable results could be
obtained. Most of the plumbing and the electrometer of the Meloy
instrument were eliminated. The Meloy sampling pump was replaced
with a source of compressed air. The output current of the photo-
multiplier tube was "fed into a linear current-voltage amplifier
having an amplification factor of 10s V/A and an output of 0 to 15 V.
The amplifier, therefore, facilitated the measurement of photo-
multiplier currents of 0 to 150 yA. A narrow band-pass filter
(394 nm) was used for the specific detection of sulfur. The heater
in the detector block was replaced with one capable of maintaining
the temperature of the block at 200°C.
A schematic diagram of the system is shown in Figure 2. Fuel
and air were supplied to the flame photometric detector at flow
rates of 140 and 200 ml/min, respectively. The airflow system was
equipped with three manually operated valves. One three-way valve -'
served as a vent to prevent a backflow of air through the sample
compartment during the loading operation. A second valve, the
sample valve, isolated the sample compartment from the airflow
system until it was appropriate to sweep the volatilized material
from the sample compartment into the flame photometric detector.
A third valve, the bypass valve, was not used routinely. Its
purpose was to block the alternate path of air to the burner and
to force all of the air through the sample compartment. The timing
involved in opening the sample valve and closing the bypass valve
was found to be critical. Improper timing resulted in the loss
of several samples because of "flame-outs"; therefore, the bypass
20
-------
VENT
VALVE .
SAMPLE (
VALVE .
>,
SEPTUM 12/5 BANDS (
. JOINT
\ r \
}
/
/BY PASS
VALVE
| J 1 ^
j HEATED TRANSFER LINE
TRANSFER ROD / —
/
HEATED SAMPLE
BLOCK (Qa.2000C)
^\
^
FLOWMETERS
f f
1
v H2 AIF
FPD (DETECTOR
BLOCK AT ABOUT
200°C)
\
RECORDER
Figure 2. Schematic Diagram of Volatilization-Flame Photometric Apparatus
-------
valve was usually left open. It was estimated that half of the air
(ca. 100 ml/min) flowed through the sample compartment when the
sample and the bypass valves were open.
Copper tubing was used to connect the air supply to the vent
valve. Stainless steel tubing was used between the bypass valve .
and the flame photometric detector. Glass or Teflon tubing was
used in the remainder of the system. The sample compartment was a
glass 12/5 ball joint fitted into a heated aluminum block and
connected to the flame photometric detector by means of a heated
Teflon transfer 'tube. Stainless steel unions were used to make
glass-to-TefIon and Teflon-to-metal joints. The temperatures of
the sample block, the transfer tube, and the detector block were
monitored by use of Chromel-Alumel thermocouples with 0°C refer-
ence junctions. These temperatures were usually between
200 and 210°C, although some experiments were done with the
sample block at 150°C.
Sample Loading Procedure
Details of the loading procedure were as follows:
• With the vent valve open and the sample valve closed,
the 12/5 ball-and-socket joint connecting the loading
compartment to the sample compartment was disconnected
and the stainless steel transfer rod was retracted
through its septum seal.
• The sample, oh a 7.3-mm diameter disk cut from a
Mitex LS filter, was placed in the end of a "carry"
tube and put into the loading compartment. The
"carry" tube was an 85-mm length of Pyrex tubing
with an o.d. of 4.5 mm and an i.d. of 3.4 mm.
• The loading compartment was then connected to the
sample compartment, the vent valve was closed, and the
hydrogen flame was ignited.
• The sample was then moved into the heated sample com-
partment with the transfer rod, and a timer was started.
• After a predetermined hold time at the test temperature,
the sample valve was opened and the volatilized material
was swept into the flame photometric detector. A hold
time of 1 min was customary at 200°C while a 2-min hold
time was used at 150°C.
\V
Calibration with Known Amounts of Sulfuric Acid -^
Initial flame photometric measurements were made with 7.3-mm
diameter sample disks cut from Mitex LS filters and spiked with
known amounts of sulfuric acid. A standard solution containing
22
-------
1.9 yg/yl of sulfuric acid in 2-propanol was applied to the Teflon
filter with a microsyringe. Standard filter disks were prepared
by putting 1.0, 1.5, or 2.0 yl of this solution on the disks and
evaporating the solvent under nitrogen. The sample disks were
stored under nitrogen until they were loaded into the sample block.
To facilitate the comparison of the results, all responses were
recorded on the 0 to 500 mV range of a Hewlett Packard Model
7128A strip-chart recorder. The chart speed was 1 in./min. The
test temperature was 200°C and the hold time was 1 min.
Although the square root of the area under the response
curves obtained with standard samples would be the most appropri-
ate parameter, the square root of peak height was taken to be a
convenient measure of the concentration of sulfuric acid. The
results of these measurements are given in Table 4.
TABLE 4. CALIBRATION .DATA FOR THE FLAME
PHOTOMETRIC DETECTOR3
Amount of
H2SOi», yg
1.9
3.8
Number of
measurements
9
3
(Peak height)*,
arbitrary units,
average
5.9
7.0
Relative
standard
deviation, %
4.1
4.7
Detector responses were obtained under the conditions
described in the text. The test temperature was 200°C.
These data do not show the expected dependence on concen-
tration; i^e_. , peak area should be proportional to the
square of sulfur mass concentration.
Tracings of typical response curves for standard samples are
shown in Figure 3. No response was obtained with blank 7.3-mm
disks that were cut from Mitex LS filters and handled in the same
manner as the standard samples except for the addition of the
sulfuric acid spike. The principal sulfuric acid peak occurred
almost immediately after the sample valve was opened. A feature
of all of the data collected with the flame photometric apparatus
was the appearance of a small secondary peak that occurred after
the sample valve had been closed for several minutes and then re-
opened. This secondary peak was attributed to the retention of
traces of sulfuric acid on the filter and the slow evolution of
this residual with time.
23
-------
200
150
>
E
g 100
o
CL
03
III
50
200 I-
150 -
STANDARD-3.8 ug of H9SO4
ON TEFLON SAMPLE DISK.
SAMPLE VALVE
OPENED
SAMPLE VALVE
.CLOSED
I
SAMPLE
VALVE OPENED
4 3
TIME, min
STANDARD - 1.9 wg OF H2S04
ON TEFLON SAMPLE DISK.
E
LU"
o 100
CL
m
OC
50
0
I
1
SAMPLE
VALVE OPENED SAMPLE
I VALVE CLOSED
J I )
©
SAMPLE VALVE
OPENED
5 4 3 2 1 0
TIME, min
Figure 3. Flame Photometric Response to Standard Samples of Sulfur ic Acid.
Temperature, 20CPC.
24
-------
Comparison of Flame Photometric and Benzaldehyde Extraction
Procedures
Several Mitex LS filters were loaded with equal amounts of
sulfuric acid aerosol. Sulfuric acid was determined on three
filters by the benzaldehyde extraction technique previously
described. The average amount of sulfuric acid on these filters
was 66 yg. Three 7.3-mm diameter sample disks were cut from
another filter for the flame photometric determination of sulfuric
acid. One disk was cut from the center of the collection filter
and two were cut from areas adjacent to the center. The effec-
tive area of the 47-mm diameter collection filter was assumed to
be the area directly over the grid of the support plate in the
filter holder. The diameter of this grid was 35 mm; therefore,
the effective area of the filter was 962 mm2 and each small
sample disk represented about 4.3% of the total effective area.
The average of square roots of the flame photometric responses
for these samples was 6.6 units. The relative standard deviation
was 2.9%. The samples were run under conditions identical to
those used for the calibration. Reference to the calibration data
given in Table 4 showed that the average amount of sulfuric acid
on the small sample disks was 2.9 yg. The total amount of sul-
furic acid on the collection filter was then calculated to be
67 yg. The agreement between the two methods was excellent:
sulfuric acid by flame photometric detection = 67 yg; sulfuric
acid by benzaldehyde extraction = 66 yg. In view of the approxi-
mation used to correlate flame photometric response with the
concentration of sulfuric acid and the possible retention of sul-
furic acid on filters, the agreement between the two different
determinations of sulfuric acid was better than expected.
25
-------
SECTION 5
RESULTS OF INTERFERENCE STUDIES
GAS AND VAPOR INTERFERENTS
Introduction of Gases Directly into Sulfuric Acid Aerosol
In the earlier stages of this project experiments were con-
ducted in which sulfur dioxide, nitrogen dioxide, or ammonia was
introduced directly into the sulfuric acid aerosol. In these
first experiments the generator was operated to give concentra-
tions of sulfuric acid of about 20 to 50 yg/m3 and the inter-
ferent gases were introduced at known concentrations into the
side arm of the sampling probe (Figure 1). Analyses for acidity
and total sulfate were made by the bromophenol blue and barium
chloranilate methods, respectively. These experiments were
generally inconclusive because of problems in measuring sulfur
dioxide concentrations in the generator gas (see page 16), the
production of nitrogen dioxide by the flame (see page 12), and
uncertainties caused by the presence of varying concentrations of
background ammonia. The results that were obtained, however, gave
some indications that neither sulfur dioxide nor nitrogen dioxide
interfered in the measurement of sulfuric acid when significant
amounts of other substances were absent.
Sulfur Dioxide. Analyses by Bromophenol Blue and Barium
Chloranilate Methods—
The most meaningful experiments with sulfur dioxide were con-
ducted by adding sulfur dioxide at a concentration of 200 yg/m3
to sulfuric acid aerosol at a concentration of about 300 yg/m3.
The results shown in Table 5 indicate that the addition of sulfur
dioxide to the aerosol apparently had little effect on the amount
of sulfuric acid collected on the filters, whether measured as
acid or as sulfate. This conclusion cannot be adequately recon-
ciled, however, with the results of other experiments which indi-
cated that sulfur dioxide is at least partly oxidized to sulfate
by the combustion products derived from the flame. One possible
explanation for no apparent oxidation of sulfur dioxide may re-
late to the lower concentration of nitrogen dioxide in the genera-
tor effluent that occurred at the lower flame temperatures used
in these experiments.
26
-------
TABLE 5. RESULTS OF EXPERIMENTS IN WHICH SULFURIC ACID
AEROSOL WAS COLLECTED ON MITEX FILTERS IN THE
PRESENCE OF SULFUR DIOXIDE3
Total H2SOi» deposited, yg
Determined as acid Determined as sulfate
Reference Reference
Run Sample filter filter Sample filter filter
No. (SO2 added) (S02 not added) (S02 added) (S02 not added) _.£-.•••- ")'
1
2
3
4
5
6
7
8
9
Av
49
-
54
-
48
50
-
47
-
50
52
-
49
-
46
45
-
50
-
58
_
55
-
55
-
-
52
-
55
54
_
58
-
60
-
-
56
-
54
57
Generator and sampling conditions: H2, 15 mmHg; 02, 103 mmHg;
Ar, 627 mmHg; aspiration rate, 3 ml/min; probe temperature,
60°C; feed solution, 2 x 10"3 N H2SOi»; sampling rate, 6 1/min;
total volume sampled, 0.120 m3. The S02 concentration was
approximately 200 yg/m3.
Acidity was determined by the bromophenol blue method. Sulfate
was determined by the barium chloranilate method.
Nitrogen Dioxide. Analyses by Bromophenol Blue and Barium
Chloranilate Methods--
These experiments were done by exposing Mitex LS filters
simultaneously to nitrogen dioxide and sulfuric acid aerosol. The
average concentration of nitrogen dioxide was 320 yg/m3 and the
average concentration of sulfuric acid was 366 yg/m3 (determined
-as S0i*~2) and 300 yg/m3 (determined as acid). About 40 yg of
sulfuric acid was collected on each filter and about 40 yg of
nitrogen dioxide was introduced into each sampling line over the
sampling period. The results are given in Table 6. They indicate
that nitrogen dioxide had no effect on the collection of sulfuric
acid. No complicating phenomena were observed in these experiments;
however, some nitrogen dioxide, in addition to that added, was
assumed to be present as a result of its formation in the flame.
27
-------
TABLE 6. RESULTS OF EXPERIMENTS IN WHICH SULFURIC ACID
AEROSOL WAS COLLECTED ON MITEX FILTERS IN THE
PRESENCE OF NITROGEN DIOXIDE5
Total H2SOt» deposited, yg
Determined as acid
Run
No.
1
2
3
4
Av
Reference
Sample filter filter
(S02 added)
_
33
-
38
36
(SO 2 not added)
_
33
-
39
36
Determined as sulfate
Reference
Sample filter filter
(S02 added)
43
-
38
—
40
(S02 not
46
-
41
-
44
added)
a Generator and sampling conditions: H2/ 15 mmHg; 02, 103 mmHg;
Ar, 672 mmHg; aspiration rate, 2.6 ml/min; probe temperature,
60°C; feed solution, 2 x 10~3 N H2SOi»; sampling rate, 6 1/min;
total volume sampled, 0.120 m3. The N02 concentration was
320 )jg/m3.
Acidity was determined by the bromophenol blue method. Sulfate
was determined by the barium chloranilate method.
Introduction of Gases and Vapors onto Filters Preloaded with
Sulfuric Acid Aerosol
Ammonia. Analyses by Bromophenol Blue and Barium Chloranilate
Methods--
The apparatus shown in Figure 4 was constructed in order to
expose filters preloaded with sulfuric acid aerosol to gaseous
interferents. Mitex filters preloaded with sulfuric acid were
put into the sample and reference filter holders. Room air was
drawn through a 40% solution of sulfuric acid, a Gelman Type AE
filter, and the sample and reference filters. The sampling rate
was 6 1/min through each filter. The solution of sulfuric acid
removed alkaline gases from the ambient air and also served to
maintain the relative humidity of the sample air at about 50%.
The function of the Gelman Type AE filter was to remove any sul-
furic acid mist that might be entrapped in the sample air. Blank
runs showed that no detectable sulfuric acid mist escaped the
Gelman filter. The interferent was introduced through a tee lo-
cated about 18 cm in front of the sample filter.
In one experiment equivalent amounts (34 yg, or 0.70 y equiv)
of sulfuric acid were deposited on each of two filters. One
filter was then exposed to sample air (at 6 1/min for 5 min) con-
taining a total of 3.5 yg (0.20 y equiv) of ammonia. The other
/
28
-------
CAST
PUMP
CRITICAL ORIFICE
(6 L/MIN)
CRITICAL ORIFICE
(6 L/MIN)
REFERENCE
FILTER
SYRINGE PUMP TO INTRODUCE
DILUTED INTERFERENT GAS
GELMANTYPE AE
GLASS-FIBER FILTER
GREENBURG-SMITH
IMPINGER CONTAINING
40% H2SO4
Figure 4. Apparatus for Exposure of Preloaded Filters to Ammonia
MIDGET
BUBBLER
(SLIP)
FILTER PRELOADED WITH
H2S04 AEROSOL
CRITICAL
ORIFICE
(1 L/MIN)
SYRINGE PUMP—^
FOR NH3
MIXING
CHAMBER
ROOM AIR
INLET
CRITICAL
ORIFICE
(1 L/MIN)
MIDGET
BUBBLER
(ASSAY)
Figure 5. Modified Apparatus for the Exposure of Preloaded Filters to Ammonia
29
-------
filter was exposed to the same sample air but without the added
ammonia. After the exposure, acidity was determined on each of
the filters by the bromophenol blue method. The reference filter
was found to contain 34 yg (0.70 y equiv) of sulfuric acid while
the sample filter (exposed to ammonia) contained 17 yg (0.35 y
equiv) of sulfuric acid. From these data it was calculated that
approximately 50% of the sulfuric acid was neutralized; the amount
neutralized, however, was greater, in terms of equivalents, than
the amount of ammonia added. This result is explainable only on
the basis of the presence of background ammonia in the 120 liters
of air sampled—which is unlikely since it had been shown that
ammonia was not present in this scrubbed air—or on the basis of
errors in measurement.
The experiment was repeated with 45 yg (0.90 y equiv) of pre-
deposited sulfuric acid and 2.5 yg (0.14 y equiv) of added ammonia.
The sample filter (exposed to ammonia) was found to contain 35 yg
(0.70 y equiv) of sulfuric acid by the bromophenol blue method.
The amount of sulfuric acid neutralized was again greater than the
amount of ammonia added, in terms of equivalents.
Additional experiments were done in which ammonia (0.14 and
0.12 y equiv) was introduced onto filters preloaded with 53 yg
(1.08 y equiv) of sulfuric acid and the filters analyzed for sul-
fate by the barium chloranilate method. As expected, the exposure
to ammonia did not affect the results obtained by determining total
sulfate.
Ammonia. Analysis by the Benzaldehyde Extraction Method--
In these experiments benzaldehyde extraction and Ba-Thorin
microtitration were used for sulfuric acid determination and the
amounts of ammonium ion retained on filters and amounts of
ammonia passing through the filters were determined. Also, the
modified apparatus depicted schematically in Figure 5 was used to
expose filters preloaded with sulfuric acid aerosol to ammonia.
In this apparatus ammonia diluted with nitrogen was added from a
syringe pump into an ambient airstream; the airstream was then
passed through a mixing chamber. The flow was split equally for
determination of ammonia concentration in one arm and for exposure
to preloaded filters and measurement of slippage through the
filters in the other arm. It was determined that a single bubbler
containing dilute sulfuric acid was sufficient for quantitative
collection of the ammonia from the airstream in either arm. The
range of ammonia concentrations used for exposure to filters pre-
loaded with sulfuric acid aerosol was 0.13 to 1.9 mg/m3 and the
sampling times were approximately 20 min at a flow rate of
1 1/min.
30
-------
The results are presented in Table 7. In terms of micro-
equivalents (y equiv) these data show a reasonably good correla-
tion between amount of sulfuric acid lost upon exposure to ammonia
and the amount of ammonium ion found on the filters. Also, slip
of ammonia through filters loaded with sulfuric acid aerosol was
significant only in the instances where the ratio of microequiva-
lent of ammonia to microequivalent of sulfuric acid exceeded 1.0.
In these instances the sulfuric acid was completely lost. In the
experiments where the amounts of ammonia were less than stoichio-
metric, the amount of sulfuric acid lost correlated fairly well
with the amount of ammonia to which the filter was exposed.
On the basis of these results it can be concluded that sul-
furic acid aerosol collected on a Mitex filter will be consumed
stoichiometrically by ammonia with the formation of ammonium
sulfate on the filter. This conclusion was evidenced by the ex-
cessive slip of ammonia after the formation of ammonium sulfate
on the filters for which the ratio of ammonia to sulfuric acid
exceeded 1.0.
Obviously, these data suggest that an alkaline gas such as
ammonia can cause a loss of sulfuric acid during the collection
of sulfuric acid aerosol on a filter.
31
-------
TABLE 7. EFFECT OF AMMONIA ON PREDEPOSITED SULFJURIC ACID
AEROSOL ON MITEX LS FILTERS
OJ
to
NH3 . . H2SOi»
added, aerosol , Ratio
y equiv added, y equiv NH3/H2S(
0
0
0
0
1
2
.15
.25
.41
.52
.50
.26
1
0
0
0
0
0
.04
.77
.80
.76
.84
.86
0.14
0.32
0.51
0.68
1.79
2.63
NH3
or NHit+ found, y equiv
« fo
)i» Filter Slip Total y
0.
0.
0.
0.
0.
1.
12
23
33
29
79
04
0.05 p. 17
0.03 0.26
0.03 p. 36
0 0.29
0.13 0.92
0.91 1.95
1
0
0
0
0
0
2$0k
und,
equiv y
.06
.53
.49
.57
E2SOn
lost,
equiv
0
0
0
0
0
0
.24
.31
.19
.84
.86
fa
b
The quantity of
allel bubbler by
ammonia exposed
the indophenol
to each
method .
filter was determined
One microequivalent
by analysis of
of" NH3 = 17 yg.
a
par-
/I
analysis of parallel filters exposed only to sulfuric acid aerosol. These analyses
were conducted with the benzaldehyde extraction and Ba-Thprin microtitration. One
microequivalent of H2SOit = 49 yg.
The ammonium ion present on the test filter was determined by extraction of one-half
of the test filter with dilute sulfuric acid and measurement by the indophenol method;
the other half was reserved for determination of residual sulfuric acid. The ammonia
that slipped the test filter was collected in a bubbler containing dilute sulfuric
acid and measured also by the indophenol method.
These results represent twice the amount of sulfuric acid found upon analysis of one-
half the test filter disk.
These data represent the difference between the sulfuric acid added and the sulfuric
acid found.
-------
Pyridine Vapor. Analysis by the Benzaldehyde Extraction Method—
Pyridine was chosen for the study of potential interference —
from organic amines. In addition to being an aromatic hetero-
cyclic compound, pyridine is a weaker base (pK, - 8.8) than
ammonia (pK, - 4.7) or typical aliphatic amines (pK, - 2 to 5) .
The experiments with pyridine vapor were conducted with the
same apparatus shown in Figure 5 that was used in identical ex-
periments with ammonia. In the present experiments benzaldehyde
extraction and Ba-Thorin microtitration were again used for
sulfuric acid determination and the amounts of pyridinium ion u,,
retained on filters and amounts of pyridine vapo~r passing through v
the filters were determined by a gas chromatographic procedure.
The gas chromatographic procedure was based on the injection of
aqueous (dilute sulfuric acid) samples containing dissolved pyri-
dine (pyridinium) onto a 1.8-m by 2-mm column packed with 5%
potassium hydroxide and 10% Carbowax 20M on a support of Gas Chrom
P, employing flame ionization detection. The range of pyridine '.':
concentrations used for exposure to filters preloaded with sul-
furic acid was 0.28 to 3.8 mg/m3 and the sampling times were
approximately 20 min at a flow rate of 1 1/min.
The results are presented in Table 8. In terms of micro-
equivalents these data show a poor correlation between the amount
of sulfuric acid lost upon exposure to pyridine vapor and the
amount of pyridinium ion (CsHsNH ) found on the filters. In most
cases, the losses of sulfuric acid were very small and the pyri-
dinium ion on the filter exceeded the loss of sulfuric acid.
These data demonstrated a much different behavior for pyridine
vapor than was found earlier for ammonia. Essentially stoichio-
metric reaction was observed for ammonia and predeposited sulfuric
acid. Because significant amounts of pyridinium ion were found on
the filters, it is speculated that the benzaldehyde extraction
removed pyridinium bisulfate (or sulfate) from the filters in
addition to residual sulfuric acid. In this event the analyses
for sulfate in the aqueous back-extract could then lead to high
results for sulfuric acid due to the soluble bisulfate. Another
explanation—although less likely—concerns the possible adsorp-
tion of unreacted pyridine vapor on the filters in the presence
of sulfuric acid. This phenomenon could conceivably result in
reporting the presence of pyridinium ion from the filter analy-
ses by gas chromatography and sulfuric acid with benzaldehyde
extraction. Both hypotheses could be tested by measuring the
extractability of authentic pyridinium bisulfate (or sulfate)
with benzaldehyde in the first case, and measuring the retention
of pyridine vapor by blank Mitex filters in the second case.
33
-------
TABLE 8. EFFECT OF PYRIDINE VAPOR ON PREDEPOSITED
SULFURIC ACID ON MITEX LS FILTERS
CsH5N H2SOi»
adde d ,
y
0
0
0
0
0
equiv
.07
.13
.17
.29
.95
CSH5N or CsHsNH"1" H2S04 H2SOit
added, w
y
l
1
1
1
1
equiv
.3
.4
.5
.3
.1
found
Filter
0
0
0
0
0
.05
.11
.25
.27
.59
, y equiv
Slip
0.02
0.06
0.03
0.06
0.40
Total
0.07
0.17
0.28
0.33
0.99
found,
y
1
1
1
0
1
equiv
.2
.5
.4
.9
.2
lost
,
y equiv
0.1
None
0.1
0.4
None
a. The quantity of pyridine to which each filter was exposed
was determined by analysis of a parallel bubbler by a gas
chromatographic method.
b. The quantity of sulfuric acid predeposited on each filter
was determined by analysis of a parallel filter exposed
only to sulfuric acid aerosol.
c. The pyridinium ion present on the test filter was determined
by extraction of one^half of the test filter with dilute
sulfuric acid and measurement by a gas chromatographic method;
the other half was reserved for determination of residual
sulfuric acid. The pyridine that slipped the test filter was
collected in a bubbler containing dilute sulfuric acid and
measured also by a gas chromatographic method.
Ammonia. Analysis by the Flame Photometric Method —
Ammonia interference was also studied with the technique
involving thermal volatilization of the sample and flame photo-
metric detection of the evolved sulfur species. In these qualita-
tive experiments, sulfuric acid was collected on a Mitex filter
from the aerosol generator, and the filter was subsequently ex-
posed to excess gaseous ammonia over a beaker containing a con-
centrated aqueous solution of ammonia. Essentially no response
was observed with the flame photometric detector upon heating
the filter that was exposed to excess ammonia; on the other hand,
the expected response was obtained for the reference filter that
was not exposed to ammonia.
34
-------
In addition, no response was obtained with the flame photo-
metric detector upon heating filters that were spliced with ammo-
nium bisulfate either from solution in methanol or as the solid
crystalline material.
These data also demonstrate that gaseous ammonia can cause
losses of sulfuric acid during sampling, and that the reaction
products, ammonium sulfate or bisulfate, do not appear to inter-
fere with determinations of residual sulfuric acid on filters by
the flame photometric technique.
Phenol Vapor. Analysis by the Benzaldehyde extraction method --
Experiments with phenol vapor were also conducted with the
apparatus shown in Figure 5 that was used for the ammonia and
pyridine studies. Bubblers containing 0.01 N NaOH were used
for determining the amount of phenol added (parallel bubbler)
and the amount of phenol that slipped the filters exposed pre-
viously to sulfuric acid aerosol (slip bubbler). The concentra-
tions of phenol in the bubblers were determined by measuring the
spectrophotometric absorbance at 287 nm due to the phenolate ion
in the alkaline bubbler solution. The analyses for sulfuric acid
were performed by the benyaldehyde extraction method.
The results of this study are given in Table 9. These data
show that little or no sulfuric acid was lost and that essentially
all of the phenol vapor slipped the filters. On the basis of
these data phenol does not appear to be a significant interfer-
ence to the sampling of sulfuric acid aerosol with Mitex filters.
PARTICULATE INTERFERENTS
Analyses by Bromophenol Blue and Barium Chlorahilate Methods
Ambient Particulate Material —
Exposure of filters with predeposited sulfuric acid to
ambient air — Each of nine Mitex LS filters was spiked with 50 yl
of a solution containing about 1.04 yg/yl of sulfuric acid.
These filters were allowed to stand in a desiccator over silica
gel for 24 hr prior to use. After "drying", all of the spiked
filters—still in the desiccator—and the necessary sampling
equipment were taken to the North Birmingham monitoring station
of the Jefferson County Health Department. Two of the spiked
filters were used to sample the air for 30 min, and two were
used to sample for 50 min. The sampling flow rate was 15.3 1/min.
The filters were placed in a desiccator over silica gel immedi-
ately after sampling. On the following day, acidity was deter-
mined by the bromophenol blue method on all of the test filters
and on three control filters. The control filters were spiked
and analyzed at the same time as the test filters. The results
35
-------
TABLE 9. EFFECT OF PHENOL VAPOR ON PREDEPOSITED
SULFURIC ACID ON MITEX LS FILTERS
A
Phenol
added, a
ug
H2SO,,
added,
Phenol
Filter
Q
found, jjcf H2SOu
Slip found,
lost,
20 65 Lost 32 64 1
82 51 <5 90 52
112 67 <5 124 68
228 77 nd 240 64 13
a. The quantity of phenol to which each filter was exposed was
determined by analysis of a parallel bubbler (0.01 N NaOH)
with a spectrophotometric method. A r
b. The quantity of sulfuric acid predeposited on each filter
was determined by analysis of a parallel filter exposed
only to sulfuric acid aerosol.
c. The amount of phenol on the filters was determined by ex-
traction with 0.01 N NaOH followed by spectrophotometric
measurement; the phenol vapor that slipped the filters was
determined by • spectrophotometric analysis of slip bubblers
containing 0.01 N NaOH.
of this experiment are given in Table 10. Although the differ-
ences in the amounts of sulfuric acid found on the test filters
and the control filters may be within experimental error for
Filters 1, 2, 3, and 4 (10- and 30-min sampling times), the
difference is certainly real for Filters 5 and 6 (60-min sampling
time) . It was concluded, therefore, that in this experiment
evidence of significant neutralization of the 54 yg of predepos-
ited sulfuric acid occurred during the sampling of approximately
1 m3 of ambient air. One factor affecting the interpretation of
these data is that the sulfuric acid could not be uniformly dis-
persed on the spiked Teflon filters. The spike consisted of
4 to 6 droplets of solution that were 1 to 2 mm in diameter. The
aqueous solution did not, of course, wet the Teflon membrane fil-
ters. Therefore, the acid was concentrated on a small portion
of the filter and most of the air that passed through the filter
did not come in direct contact with the deposited acid. One would
expect the neutralization of the acid by alkaline materials in the
atmosphere to be more pronounced had the filters been spiked uni-
formly with sulfuric acid aerosol particles.
36
-------
TABLE 10. EFFECT OF AMBIENT AIR ON
SULFURIC ACID-SPIKED FILTERS
Sample
Control
Control
Control
1
2
3
4
5
6
Volume--
sampled,
m3
0
0
0
0.153
0.153
0.459
0.459
0.918
0.918
Amount of
HaSOii found,
yg
55
53
55
55
58
53
53
42
38
Amount of HaSOitOn test
filter minus amount on
control filter k
_
-
—
+ 1
+ 4
-1
-1
-12
-16
A.
a. Determined by the measurement of acidity by the bromophenol
blue method.
b. The average amount of sulfuric acid found on the control
filters was 54 yg. The calculated amount deposited was
52 yg.
Exposure of filters with predeposited ambient particulate
material to sulfuric acid aerosol—Particulate material was col-
lected on Mitex LS filters by sampling air for several hours at
a location near a busy street. The filters were weighed to deter-
mine the amount of particulate material collected. Some of the
filters were then exposed to sulfuric acid aerosol from the gen-
erator for the period of time required to deposit about 50 .yg of
sulfuric acid; others were not exposed to the acid. Reference
filters, containing no ambient particulate material, were simul-
taneously exposed to the same sulfuric acid aerosol as the sample
filters. The filters were then analyzed by the bromophenol blue
or barium chloranilate methods. The results are given in Table
11. Analyses for total sulfate showed that all of the added sul-
furic acid was recovered along with the soluble sulfate in the
ambient particulate material. However, measurements of acidity
indicated that only about 50% of the expected amount of acid was
present. Therefore, on the basis of these data, it appeared that
a quantitative conversion of sulfuric acid to an equivalent
amount of soluble bisulfate occurred.
37
-------
TABLE 11. EFFECT OF PREDEPOSITED AMBIENT PARTICULATE UPON
THE COLLECTION OF SULFURIC ACID AEROSOL ON
MITEX LS FILTERS
Ambient
particulate
on filter,
mga
1.7
0.9
0.7
1.4
1.0
2.1
H2SOi» Total H2SOi»
aerosol K found, ygc
added ,
none
62
none
none
50
56
yg~ as SOt*
128
127
-
—
—
~^ as H+
nde
nd
20
35
H2SOit found
less S0it~2
from
particulate, Recovery
yg %
59d 95
-
40
63
A
a. This r
>articulc
ite material
was s ample<
d from the ambient air
before the addition of the sulfuric acid aerosol.
b. The amount of sulfuric acid aerosol added to each filter was
determined by analysis of a parallel filter exposed only to
sulfuric acid aerosol.
c. Sulfate was determined by the barium chloranilate method and
acid was determined by the bromophenol blue method.
d. This number resulted from the following calculation:
127 - 128 (0.9/1.7).
e. nd = not detected.
Calcium Carbonate —
An aerosol of finely powdered calcium carbonate was generated
with a DeVilbiss powder blower and about 1 to 3 mg of the dispersed
powder was collected on a Mitex filter upon which about 50 yg of
sulfuric acid had been predeposited. The results of the analyses
of these filters are shown in Table 12. A complete loss of both
acidity and soluble sulfate was observed. This result suggested
that the predeposited sulfuric acid reacted completely with the ex-
cess calcium carbonate forming insoluble calcium sulfate.
38
-------
TABLE 12. EFFECT OF COLLECTED CALCIUM CARBONATE AEROSOL
UPON PREDEPOSITED SULFURIC ACID
ON MITEX LS FILTERS
CaCOs
aerosol
added, mg
1.4
3.1
1.3
3.5
0.8
3.3
2.6
3.1
H2SOlt
aerosol .
added, yg
none
none
68
62
none
none
44
65
r*
HaSOij found, yg~ Recovery,
''as SOi,-2
ndd
nd
0.3
0.4
-
-
-
~
as H+
_
-
-
-
nd
nd
nd
nd
%
-
-
<1
<1
-
-
0
0
a. The amount of calcium carbonate was determined by weighing.
The calcium carbonate aerosol was added immediately follow-
ing the collection of the sulfuric acid aerosol and the total
sampling time was approximately 20 to 30 min.
b. The amount of sulfuric acid aerosol added to each filter
was determined by analysis of a parallel filter exposed only
to sulfuric acid aerosol.
c. Sulfate was determined by the barium chloranilate method and
acid was determined by the bromophenol blue method.
d. nd = not detected.
Ferric Oxide —
An experiment similar to that described above for calcium car-
bonate was conducted with a dispersed fine powder of ferric oxide.
A small but significant loss of acidity was observed, as shown in
Table 13. The average recovery of sulfuric acid, as measured by
the bromophenol blue method, was approximately 72%. The average
recovery of total added sulfate, as measured by the barium chlor-
anilate method, was substantially higher, approximately 94%. Thus,
it appeared that some loss of the predeposited acid occurred, with
the formation of an equivalent amount of soluble bisulfate.
39
-------
TABLE 13. EFFECT OF COLLECTED FERRIC OXIDE AEROSOL UPON
PREDEPOSITED SULFURIC ACID ON MITEX LS FILTERS
FeaOa H2SOi» ^
aerosol aerosol , H2SOi» found, yg Recovery,
added, mg added, yg as S0i+~z as H+
0.4
0.6
0.3
0.8
2.0
0.1
0.4
0.4
0.5
0.5
0.4
0.4
1.5
none
none
28
37
80
none
none
none
none
20
22
66
78
ndd
nd
24
36
80
-
-
-
-
-
-
-
~
^_
-
-
-
-
nd
nd
nd
nd
8
16
62
62
-
86
97
100
-
-
-
-
40
73
94
79
'I a. The amount of ferric oxide was determined by weighing. The
ferric oxide aerosol was added immediately following the
collection of the sulfuric acid aerosol and the total
sampling time was approximately 20 to 30 min.
b. The amount of sulfuric acid aerosol added to each filter
was determined by analysis of a parallel filter exposed only
to sulfuric acid aerosol.
c. Sulfate was determined by the barium chloranilate method
and acid was determined by the bromophenol blue method.
Analyses by the Benzaldehyde Extraction Method
Ambient Particulate Material--
Filters with predeposited ambient particulate material were
exposed to sulfuric acid aerosol as described earlier, with analy-
sis, however, by the benzaldehyde extract!on-barium Thorin titra-
tion procedure. The results in Table 14 show a slight contribu-
tion of sulfate (as HaSOi,) from the particulate material itself;
however, the results were below the reproducible limit of detec-
tion (reported as <5 yg). The recoveries of spikes added to
extracts were adequate (79 and 88%), but the losses of sulfuric
acid added to the preloaded filters as aerosol were very signifi-
cant. The recoveries of sulfuric acid from the filters were
40
-------
variable and ranged from 29 to 38%. These results were not
corrected for the residual sulfate from the particulate, which
could have been significant because of the somewhat larger amounts
of particulate present on the test filters as opposed to the blank
filters. Therefore, the recoveries could possibly have been even
lower than those reported here.
These results are in sensible agreement with those reported
earlier in that the formation of bisulfates or possibly sulfates
from sulfuric acid appears to take place readily on a filter pre-
loaded with ambient particulate material.
TABLE 14. EFFECT OF PREDEPOSITED AMBIENT PARTICULATE
MATERIAL UPON SULFURIC ACID COLLECTED
ON MITEX LS FILTERS
/ Ambient
particulate
added , mga
- 0.3
0.4
1.7
1.2
1.8
1.9
2.1
H2SO,
added, jjg
none
none
none
none
(lost)
24
34
H2SOi»
spike, ngc
none
none
58
58
none
none
none
H2SOi»
found, yg
<5
<5
46
51
5
9
10
Recovery
%
mmi
-
79
88
-
38
29
a.
b.
c.
The amount of ambient particulate material was determined
by weighing. The particulate was collected in the vicinity
of a busy roadway in Birmingham over a 15-hr period.
The amount of sulfuric acid added to each filter was deter-
mined by analysis of a parallel filter exposed only to
sulfuric acid aerosol. The sampling time was 20 to 30 min.
Analytical spikes of sulfuric acid in benzaldehyde were
added to some of the filter.extracts that contained suspended
ambient particulate.
Calcium Carbonate--
The results of the analyses for sulfuric acid with benzalde-
hyde extraction that were performed on filters loaded with calcium
carbonate are presented in Tables 15 and 16. The data in Table
15 are for exposures of predeposited sulfuric acid aerosol to
calcium carbonate aerosol and the data in Table 16 are for
41
-------
TABLE 15. EFFECT OF CALCIUM CARBONATE AEROSOL UPON
PREDEPOSITED SULFURIC ACID ON MITEX LS FILTERS
/ CaC03
aerosol
added, mg
1 1.3
1.6
2.9
2.6
2.8
2.8
2.9
3.9
3.3
2.1
2.2
5.0
3.9
3.6
2.2
3.0
aerosol ,
added, yg
none
none
none
none
45
49
48
none
none
none
none
none
none
47
47
47
spike
added, yg
none
none
none
none
none
none
none
53
53
53
53
53
53
53
53
53
found, yg
nde
nd
nd
nd
11
12
11
50
56
50
50
39
40
64
64
61
Recovery, %
—
—
—
24
24
23
94
106
94
94
74f
75f
23g
23g
17g
a. The amount of calcium carbonate was determined by weighing.
The calcium carbonate aerosol was added immediately follow-
ing the collection of the sulfuric acid aerosol and the
total sampling time was approximately 20 to 30 min.
b. The amount of sulfuric acid aerosol added to each filter was
determined by analysis of a parallel filter exposed only to
sulfuric acid aerosol.
c. Analytical spikes of sulfuric acid were added to some of the
filter extracts.
d. All analyses for sulfuric acid were conducted with the benz-
aldehyde extraction technique and the Ba-Thorin microtitration.
e. nd = not detected
f. These results are for sulfuric acid spikes added to benzalde-
hyde extracts containing suspended calcium carbonate particu-
late.
g. These recoveries are calculated for the sulfuric acid aerosol
assuming the spikes were recovered with 100% efficiency.
42 i «'
-------
TABLE 16. EFFECT OF PREDEPOSITED CALCIUM CARBONATE UPON
SULFURIC ACID AEROSOL COLLECTED ON MITEX LS FILTERS
CaC03
aerosol
added, mga
3.6
5.2
5.0
2.5
2.4
none
none
HaSO,^
aerosol ,
added, yg
56
56
57
none
none
none
none
H2S04
spike
added, ygc
none
none
none
53
53
53
53
^2 \ d
found, yg
6
8
15
36
38
46
47
Recovery,
11
14
26
68e
72e
87!
89
a. The amount of calcium carbonate was determined by weighing.
The calcium carbonate aerosol was added prior to the collec-
tion of sulfuric acid aerosol and the total sampling time
was 20 to 30 min.
b. The amount of sulfuric acid aerosol added to each filter was
determined by analysis of a parallel filter exposed only to
sulfuric acid aerosol.
c. Analytical spikes of sulfuric acid in benzaldehyde were added
to some of the filter extracts.
d. All analyses for sulfuric acid were conducted with the benz-
aldehyde extraction technique and the Ba-Thorin microtitra-
tion.
e. These results are for sulfuric acid spikes added to benzalde-
hyde extracts containing suspended calcium carbonate particu-
late.
f. These data are typical values for the recovery of sulfuric
acid from standard solutions in benzaldehyde.
exposures of sulfuric acid aerosol to predeposited calcium carbon-
ate. The data in both tables demonstrate the favorable recovery
of analytical spikes of sulfuric acid in the workup of the samples;
also, matrix effects are shown to be negligible. However, it is
evident from these results that a marked loss of sulfuric acid
occurred on the filters in the presence of collected calcium car-
bonate aerosol. Whether the sulfuric acid aerosol was added be-
fore or after the collection of calcium carbonate aerosol, losses
of sulfuric acid generally in excess of 75% were observed (i«e_. /
43
-------
recoveries were typically less than 25%). It can be speculated
that the losses of sulfuric acid were the result of reaction of
the acid with calcium carbonate to form calcium sulfate. Because
of the possibility of formation of this product, experiments were
conducted in which it was demonstrated that calcium sulfate was
not extracted from filters with benzaldehyde and therefore, did
not interfere with the quantitation of sulfuric acid. These
results are in general agreement with those described previously
in which the less specific bromophenol blue method for the deter-
mination of sulfuric acid was used. Both investigations show that
a potential problem exists in sampling sulfuric acid aerosol
simultaneously with an alkaline reactant such as calcium carbonate,
Ferric Oxide—
The experiments to investigate the influence of particulate
ferric oxide upon the stability of collected sulfuric acid aerosol
were performed in a manner comparable to those described for par-
ticulate calcium carbonate. The results are presented in Table 17.
These data also point out the favorable recovery of analytical
spikes and the apparent freedom from matrix effects in the deter-
mination of sulfuric acid in the presence of ferric oxide particu-
late. Upon analysis of filters exposed to sulfuric acid aerosol
following collection of ferric oxide particulate, insignificant
losses of sulfuric acid were observed. Indeed, approximately 95%
of the added sulfuric acid was found to remain unchanged on the
filters loaded with ferric oxide particulate. This result concurs
roughly with data obtained on ferric oxide interference with the
less specific bromophenol blue method. The results of both of
these investigations indicate that ferric oxide particulate does
not pose a severe interference in the collection and quantitation
of sulfuric acid aerosol.
Fly Ash—
The experiments to investigate the influence of fly ash upon
the stability of sulfuric acid collected on Mitex filters from an
aerosol were performed in a manner comparable to the experiments
described for the particulates ferric oxide and calcium carbonate.
In the present experiments, fly ash was deposited on Mitex filters
from an aerosol before the collection of sulfuric acid. The
sample of fly ash employed in these experiments was obtained from
the electrostatic precipitator of a coal-fired power plant and was
reported to contain 0.5% of water-soluble sulfate.
The results of the study of fly ash interference are pre-
sented in Table 18. These data show that no contribution from
residual sulfate in the fly ash was detected in the analyses with
the benzaldehyde extraction technique. Also, spikes of sulfuric
acid in benzaldehyde that were added to filter extracts in the
presence of suspended fly ash particulate were recovered with
44
-------
TABLE 17. EFFECT OF PREDEPOSITED FERRIC OXIDE
UPON SULFURIC ACID AEROSOL COLLECTED
ON MITEX LS FILTERS
Fe203
aerosol
added, mg
0.3
0.5
0.3
0.4
0.2
0.3
0.5
none
none
aerosol ,
added, yg
none
none
52
45
43
none
none
none
none
spike
added, yg
none
none
none
none
none
43
43
43
43
found, yg
nd
nd
52
43
39
36
34
39
35
Recovery, %
-
-
100
96
91
84e
79e
91
81
The amount of ferric oxide was determined by weighing. The
ferric oxide aerosol was added prior to the collection of sul-
furic acid aerosol and the total sampling time was 20 to 30
min.
The amount of sulfuric acid aerosol added to each filter was
determined by analysis of a parallel filter exposed only to
sulfuric acid aerosol.
Analytical spikes of sulfuric acid in benzaldehyde were added to
some of the filter extracts.
All analyses for sulfuric acid were conducted with the benzalde-
hyde extraction technique and the barium Ba-Thorin microtitra-
tion.
These results are for sulfuric acid spikes added to benzaldehyde
extracts containing suspended ferric oxide particulate.
45
-------
TABLE 18. EFFECT OF PREDEPOSITED FLY ASH UPON SULFURIC ACID
COLLECTED ON MITEX LS FILTERS
Fly ash H2SOi» i H2SOif H2SOi»
added, mga added, yg spike, yg° found, yg Recovery
_
4.
3.
1.
1.
1.
1.
2.
3
4
5
5
8
5
8
none
none
none
none
40
38
38
none
none
43
43
none
none
none
ndd .
nd
36
38
32
28
31
•
-
84
88
80
74
82
The amount of fly ash was determined by weighing. The fly ash
was collected prior to the collection of sulfuric acid and the
total sampling time was 20 to 30 min.
The amount of sulfuric acid added to each filter was determined
by analysis of a parallel filter exposed only to the sulfuric
acid aerosol.
c Analytical spikes of sulfuric acid in benzaldehyde were added
to some of the filter extracts that contained suspended fly ash
particulate.
nd = not detected.
efficiencies of 84 and 88%. (Analyses of standard solutions of
sulfuric acid in benzaldehyde generally result in recoveries of
80 to 90%.) The high recovery of sulfuric acid spikes indicated
that sulfuric acid was not lost in the analytical work-up of the
filter samples.
Upon addition of sulfuric acid to filters preloaded with fly
ash it was found on analysis that essentially all of the sulfuric
acid was recovered. The recovery of sulfuric acid from three
filters averaged 79% and it appeared that this particular fly ash
sample, caused no substantial degradation of the collected sulfuric
acid.
46
-------
Although this appears to be a significant result for fly ash,
we cannot at this time predict whether or not this would be the
general result for variety of diverse fly ash samples. An addi-
tional uncertainty in these experiments with fly ash concerns the
possibility of benzaldehyde extraction of bisulfates or sulfates
resulting from reaction of the fly ash with the added sulfuric
acid aerosol. In several studies concerning the potential inter-
ferences posed by pure reagents such as calcium carbonate and
ammonia, the possibility of nonselective extraction of the corre-
sponding bisulfates or sulfate was investigated with known spikes
of the authentic materials on filter disks. However, the complex-
ity of fly ash precludes the measurement of the extractability of
all of the possible sulfate and bisulfate compounds that could be
formed on reaction of the fly ash with sulfuric acid. On the
other hand, an analytical result which indicates that a loss of
sulfuric acid occurred on a filter in the presence of a predepos-
ited interferent (and not in the analytical workup) not only
identifies an active interference to collection of sulfuric acid
but also demonstrates that coextraction of bisulfates and sulfates
into benzaldehyde did not take place in the analysis for residual
sulfuric acid. Such a result is demonstrated by the data from the
study of the effect of ambient particulate material on sulfuric
acid.
Soot—
Experiments to study the influence of soot (amorphous carbon)
on the stability of collected sulfuric acid were performed in a
manner comparable to other experiments with particulate interfer-
ents. The soot was predeposited onto tared filters from a fuel-
rich acetylene-air flame and, after collection of sulfuric acid,
the filters were analyzed with the usual benzaldehyde extraction
technique. The results of these experiments are given in Table 19,
These data also show no sulfate contribution from the soot and
demonstrate adequate recovery of spikes (79 and 81%) in the pres-
ence of the soot. The recovery of sulfuric acid collected on the
soot-loaded filters averaged about 81%. Therefore, under these
experimental conditions, soot was not found to be detrimental to
the stability of sulfuric acid collected on the preloaded filters.
Silicate Dust—
Efforts to determine the effect of finely ground concrete on
the determination of sulfuric acid predeposited on a filter were
not successful when the analysis was attempted by the benzaldehyde
extraction method. The strong alkalinity of the concrete dust
appeared to interfere in the analysis. Another experiment was run
with a different silicate dust, a fire clay from Golden, Colorado.
The results in Table 20 indicate some apparent loss of acid
occurred, particularly with the larger amounts of the clay.
47
-------
TABLE 19. EFFECT OF PREDEPOSITED SOOT UPON SULFURIC ACID
COLLECTED ON MITEX LS FILTERS
Soot
added , mg
1.2
1.1
0.9
0.9
0.9
0.9
0.7
added, yg
none
none
none
none
40
35
34
spike, ygc
none
none
58
58
none
none
none
found, yg
ndd
nd
46
47
33
28
27
Recovery, %
-
-
79
81
83
81
80
The amount of soot was determined by weighing. The soot was
collected from an aerosol prior to the collection of sulfuric
acid and the total sampling time was 20 to 30 min.
The amount of sulfuric acid added to each filter was determined
by analysis of a parallel filter exposed only to the sulfuric
acid aerosol.
Analytical spikes of sulfuric acid in benzaldehyde were added to
some of the filter extracts that contained suspended soot
particulate.
nd = not detected.
48
-------
TABLE 20. EFFECT OF PREDEPOSITED CLAY UPON SULFURIC ACID
COLLECTED ON MITEX LS FILTERS
Clay
added , mg
1.8
2.6
1.2
2.9
0.9
1.2
2.2
added, yg
none
none
none
none
52
67
43
spike, ygc
none
none
62
62
none
none
none
' found, yg
<5
<5
55
50
46
51
23
Recovery, %
-
-
89
81
88
76
53
The amount of clay was determined by weighing prior to the
collection of sulfuric acid aerosol.
The amount of sulfuric acid added was determined by analysis of
a parallel filter exposed only to the sulfuric acid aerosol.
Analytical spikes of sulfuric acid in benzaldehyde were added
to some of the filter extracts that contained suspended clay.
Analyses with a Flame Photometric Detector
Ambient Particulate Material—
Sample heated to 200°C--Tracings of flame photometric re-
sponses of sample disks cut from a filter loaded with 0.5 mg of
ambient particulate matter, a reference filter loaded with about
50 yg of sulfuric acid deposited from an aerosol, and a filter
loaded with 0.5 mg of ambient particulate matter plus about 50 yg
of sulfuric acid aerosol are shown in Figure 6. These experiments
were done under the following conditions: range, 0-500 mV; chart
speed, 1 in./min; temperature, 200°C; hold time, 1 min. Sulfuric
acid was collected from the output of the laboratory aerosol
generator during the simultaneous exposure of a clean filter
(reference) and a filter preloaded with ambient particulate matter
(sample). The filters were mounted in parallel sampling lines
and samples were collected for 20 min at the rate of 6 1/min.
Equal amounts of sulfuric acid were thus deposited on each of the
filters. The ambient particulate matter was collected at a
sampling station located in the vicinity of the Institute. This
station was about 5 m from a busy street. The total volume of air
sampled was about 10 m3. The filters were stored in a desiccator
until they could be analyzed.
49
-------
50
03
O
0-
tn
UJ
CC
H2S04 AEROSOL PLUS AMBIENT PARTICULATE
MATTER ON TEFLON SAMPLE DISK. THIS
DISK WAS CUT FROM THE SAMPLE FILTER.
©
SAMPLE
VALVE OPENED
TIME, min
200 r—
150 -
H2S04 AEROSOL ON TEFLON
SAMPLE DISK. THIS DISK
WAS CUT F.ROM THE REFERENCE
FILTER.
RESPONSE. mV
So
0
SAMPLE VALVE CAMPLE VALVE
OPENED OPENEDv
/SAMPLE VALVE N. /
CLOSED X
\, ^
1 1 f , 1 1 1
I
^
6 5 ' ' 3 2 1 0
TIME, min
RESPONSE, mV
0 g
AMBIENT PARTICULATE
TEFLON SAMPLE DISK.
1
5
I
4
MATTER ON
1
3
TIME, min
©SAMPLE VA
OPENED
-; — N^-,
2 1 0
Figure 6. Effect of Predeposited Ambient Particulate Matter on Flame Photometric
Detection of Sulfuric Acid. Temperature, 20CPC.
50
-------
Figure 6A shows the flame photometric response to ambient
particulate matter alone. The initial peak may or may not be due
to ambient sulfuric acid. The more gradual response followed by
the very slow decay of the signal is assumed to be due to sulfur
compounds that are more difficult to volatilize then sulfuric acid
or that are slowly liberated at the elevated temperature. The
flame photometric response to sulfuric acid alone is shown in
Figure 6B. This response is similar to those obtained with stan-
dard samples of sulfuric acid shown in Figure 3. The total amount
of sulfuric acid on the reference filter, calculated from the
flame photometric response, was 54 yg. This is about the amount
expected based on the operating parameters of the sulfuric acid
aerosol generator. Figure 6C shows the flame photometric response
to a disk cut from the sample filter that was loaded with ambient
particulate matter and sulfuric acid. There is a marked decrease
in the magnitude of the peak that is attributed to sulfuric acid.
The gradual increase and slow decay of signal following the initial
peak is, again, assumed to be due to difficulty volatilized sulfur
compounds. Since the calibration data in Table 3 did not cover
the range for amounts of sulfuric acid significantly less than 1 yg,
it is not feasible to estimate the sulfuric acid concentration
represented by this peak.
Sample heated to 150°C—The curves in Figure 7 show the flame
photometric responses to samples of sulfuric acid, ambient partic-
ulate matter, and ambient particulate matter plus sulfuric acid
at 150°C. The load of ambient particulate matter on each of the
two preloaded filters was 0.5 mg. The nominal amount of sulfuric
acid on the reference and sample filters was 50 yg. The transfer
line and the detector block remained at 200°C. Unlike the experi-
ments with the sample block at 200°C (Figure 6), a 2-min hold time
was used in the experiments at 150°C. In other respects—methods
of sample collection, air and hydrogen flow rates, and recording
conditions—the experiments at 150°C were identical to thosev at
200°C. No attempt was made to quantitate the results of the
experiments at 150°C; however, the curves do show that the magni-
tude of the responses for standard samples deposited from a syringe
and for filter samples deposited from an aerosol were much smaller
at 150°C than at 200°C. Figures 7B and 7D are responses for
samples of ambient particulate matter and ambient particulate
matter plus sulfuric acid. The curves are almost identical, il-
lustrating the difficulty of recovering sulfuric acid at 150°C
from filters containing ambient particulate matter.
Effect of hold time—Figure 8 illustrates, the effect of hold
time at 200°C on the response to samples of ambient particulate
matter plus sulfuric acid deposited from an aerosol. These
samples were 7.3-mm diameter disks cut from a Mitex LS filter that
was preloaded with 0.5 mg of ambient particulate matter and then
loaded with about 50 yg of sulfuric acid. The gas flows and re-
cording conditions were the same as those previously described.
The curves show that the magnitude of the initial peak increased
51
-------
> 50
£
111"
03
0
Q.
CO
UJ
H2S04 AEROSOL PLUS AMBIENT PARTICULATE MATTER
ON TEFLON SAMPLE DISK. THIS DISK WAS CUT
FROM THE SAMPLE FILTER.
CO C
1 i 1
o
J
1 1 1 1 | |
5432
TIME, min
> 50
uf
OJ
Z
o
D.
00
UJ
pr n
U- U
H2S04 AEROSOL ON TEFLON SAMPLE
DISK. THIS DISK WAS CUT FROM THE
REFERENCE FILTER.
Ao c
-J\\ *
_ 1_ l i l
5432
TIME, min
> 50
UJ
Z
O
Q-
03
C 0
AMBIENT PARTICULATE MATTER ON
TEFLON SAMPLE DISK.
1 0 •
©
Jlr
i i
1 0
V )
0
J
1 1 1 1 1 l
5432
TIME, min
50 •—
E
uj"
RESPONS
o
STANDARD- 1.9 jug OF H2SO4 ON TEFLON
SAMPLE DISK.
O CO
. — xvi — /t__ L'Vl
i 1 1 i i i
it
1 0
0
i JL
1 J
i n
TIME, min
Figure 7. Effect of Predeposited Ambient Paniculate Matter on Flame Photometric
Detection of Sulfuric Acid. Temperature, 150P C.
O = Sample Valve Opened; c = Sample Valve Closed
52
-------
100 -
©
10-MIN HOLD AT 200°C
-------
and the broad band following the initial peak decreased as the
hold time was increased from l min to 5 min (Figures 8A and 8B).
This observation suggested that difficultly volatilized or slowly
decomposing sulfur compounds may contribute to the initial peak if
the hold time is too long at 200°C. Although there was not a
drastic increase in the size of the peak (area) between the 5-min
hold and the 10-min hold at 200°C, the "hump" in the curve dis-
appeared (Figures 8B and 8C).
Calcium Carbonate—
The results obtained with the flame photometric measurements
agreed with similar experiments conducted with the benzaldehyde
extraction technique. That is/ the sulfuric acid collected on a
filter was lost upon exposure to particulate calcium carbonate
whether the acid was collected before or after the deposition of
the calcium carbonate. This result is demonstrated in Figures 9,
10, and 11. Figure 9B shows the slight response obtained with
calcium carbonate alone, probably due either to an impurity in
the calcium carbonate or to residual sulfur in the apparatus.
54
-------
£ 50 -
111
CO
O
CL
CO
Ul
cc
1 1 1
CaCO3 ON TEFLON DISK CUT FROM
SAMPLE FILTER PRELOADED WITH
.2.2 mg OF CaCO3
'
SAMPLE VALVE
OPENED
J
' 1
1 1
SAMPLE
VALVE CLOSED
I
1 1
©
SAMPLE
VALVE
OPENED
0 —
TIME, min
200
©
150
STANDARD -1.9 ug OF H2S04
ON TEFLON SAMPLE DISK
u
CO
§ 100
0.
CO
111
cc
50
SAMPLE VALVE
OPENED
SAMPLE VALVE
CLOSED
SAMPLE
VALVE
OPENED
TIME, min
Figure 9. Flame Photometric Response to Sulfuric Acid Alone and to Calcium
Carbonate Alone. Temperature, 200PC.
55
-------
50
CO
O
a.
CO
1
_
CaCO3 DEPOSITED ON
FILTER PRELOADED
WITH H2SO4
1
1 1
SAMPLE VALVE
OPENED
wJ
1 1
1 1
SAMPLE VALVE
CLOSED
i ,
1 1
-
SAMPLE
VALVE
OPENED
L
TIME, min
200
H2SO4 AEROSOL ON TEFLON
SAMPLE DISK. DISK CUT FROM
REFERENCE FILTER LOADED
WITH ABOUT 50 ug OF H2SO4
AEROSOL
150
o
CO
O
a.
CO
100
50
SAMPLE
VALVE CLOSED
SAMPLE
VALVE OPENED
SAMPLE
VALVE
OPENED
TIME, min
Figure 10. Effect of Calcium Carbonate on Flame Photometric Detection of
Predeposited Sulfuric Acid. Temperature, 200°C.
56
-------
ou
E
UJ*
CO
2
CO
LU
CC
o
1
H2SO4 DEPOSITED ON
FILTER PRELOADED
WITH CaCO3
1
5
1 1
SAMPLE VALVE
OPENED
v
1 1
4 3
TIME, min
|
SAMPLE VALVE
CLOSED v
\
1
2
1
SAMPLE VALVE
OPENED
\©
1
1 0
150
E 100
LU"
CO
O
a.
V)
50
H2SO4 AEROSOL ON TEFLON SAMPLE
DISK. DISK CUT FROM REFERENCE
FILTER LOADED WITH ABOUT 50 U9 OF
H2SO4 AEROSOL
SAMPLE VALVE
OPENED
SAMPLE
VALVE CLOSED
•it-
©
SAMPLE VALVE
OPENED
TIME, min
Figure 11. Effect of Predeposited Calcium Carbonate on Flame Photometric
Detection of Sulfuric Acid. Temperature, 200°C.
57
-------
REFERENCES
1. Thomas, R.L., V. Dharmarajan, and P. W. West. Convenient
Method for Generation of Sulfuric Acid Aerosol. Environ.
Sci. Technol., 8(10) :930-935 , 1974.
2. West, P.W., and J.J. Chaing. Spectrophotometric Determina-
tion of Atmospheric Acidity by Means of the Displacement of
the Equilibrium of Acid-Base Indicators. J. Air Poll.
Control Assoc., 24 (7) :671-673, 1974.
3. Bertolacini, R.J., and J.E. Barney II. Colorimetric Deter-
mination of Sulfate with Barium Chloranilate. Anal. Chem.,
29(2):281-283, 1957.
4. Schafer, H.N.S. An Improved Spectrophotometric Method for
the Determination of Sulfate with Barium Chloranilate as
Applied to Coal Ash and Related Materials. Anal. Chem.,
39(14):1719-1726, 1967.
5. Leahy, D., R. Siegel, P. Klotz, and L. Newman. The Separa-
tion and Characterization of Sulfate Aerosol. Atmos.
Environ., 9(2):219-229, 1975.
6. Fritz, J.S., and S.S. Yamamura. Rapid Microtitration of
Sulfate. Anal. Chem., 27(9):1461-1464, 1955.
7. Harwood, J.E., and A.L. Kuhn. A Colorimetric Method for
Ammonia in Natural Waters. Water Research, 4(12):805-811,
1970.
8. Tentative Method of Analysis for Nitrogen Dioxide Content
of the Atmosphere (Griess-Saltzman Reaction). In: Methods
of Air Sampling and Analysis, American Public Health Associa-
tion, Washington, D.C., 1972. pp. 329-336.
9. Tentative Method of Analysis for Sulfur Dioxide Content of
the Atmosphere (Colorimetric). In: Methods of Air Sampling
and Analysis, American Public Health Association, Washington,
D.C., 1972. pp. 447-455.
10. Liu, B.Y.H., and K.W. Lee. Efficiency of Membrane and
Nuclepore Filters for Submicrometer Aerosols. Environ. Sci.
and Technol., 10(4):345-350, 1976.
58
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-027
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
DEVELOPMENT OF A PORTABLE DEVICE TO COLLECT
SULFURIC ACID AEROSOL
Interim Report
5. REPORT DATE
February 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
William J. Barrett, Herbert C. Miller,
Josiah E. Smith, Jr., and Christina H. Gwin
8. PERFORMING ORGANIZATION REPORT NO.
Project 3533-XII
SORI-EAS-76-397
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
1AA601
11. CONTRACT/GRANT NO.
68-02-2234
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, N. C. 27711
13. TYPE OF REPORT AND PERIOD COVERED
Interim 6/75-5/76
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The purpose of this investigation was to determine the effects of possi-
ble atmospheric interferents on the quantitative collection of sulfuric
acid aerosol on a filter. Sulfuric acid aerosol was generated in the
laboratory with a flame atomizer and collected on Teflon filters. The
filters were exposed to potential gas and vapor interferents and to
particulate interferents during, before, or after the collection of the
sulfuric acid. Measurements of sulfuric acid were made by an acid-base
indicator method or by extraction with benzaldehyde and titration.
Also, sulfur evolved on heating the filters was measured by the flame
photometric method. Ammonia, particulate calcium carbonate, and ambient
particulate material (collected near a busy street) caused severe losses
of sulfuric acid; particulate ferric oxide and silicate clay caused an
intermediate loss; pyridine and phenol vapors, particulate fly ash, and
soot caused little or no loss; and sulfur dioxide and nitrogen dioxide
had no effect (in the absence of other materials).
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATl Field/Group
*Air pollution
*Sulfuric acid
Aerosol
Collecting methods
*Filters
Tests
Atmospheric
interferents
13B
07B
07D
14B
13. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
67
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
59
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